Hi Dederik. Don't know if you've seen this article but heard it recommended as the definitive guide on tuning lug rigs: https://www.storerboatplans.com/tuning/lug-rig-setup/everything-lug-rig-and-lug-sail/

   I think the anwer is that yes, you'll want a system (apart from the halyard and downhaul) that holds both the yard and boom agains the mast.  Not only does it help with sail shape and therefore sailing performance (especially when the yards are on the leward side of the mast), but it also helps controll the mess when hoisting and lowering sails.  And having the upper yard captured to the mast is almost required if you're going to reef the mainsail, as without it, the long length of halyard will allow a large gap between the yard and the mast on the "off" tack, no matter how much downhaul tension you use.  While there is no requirement, I think the nautical way of rigging is with the lug sail hoisting on the port side of the mast - as that gives the best sail shape (unaffected by the mast) when on the starboard tack - and for right of way reasons the starboard tack is a "preferred" tack.  I'm not sure if there is any other reason, other than for sailors that sail different boats  they can always expect rigging to be consistent.

There are very many methods to accomplish the capture - so I'll leave it to you to research what others have done, and what looks best to you.  A system that is quick and easy - meaning doesn't require tying knots every time you rig the sails - or simply lower them for the short row into or out of harbor or away from the dock to hoist sails, or to row when the wind dies - is advisable.

And as an extra "hint" - because you're going to be forcing even more contact between the yards and the mast that might happen otherwise, some protection between the two is advisable.  Might be fancy leather wrap like on oars.  Might be a simple tight wrap of tarred marlin like I used, which works very well.  Make the protected area on the boom and yard longer than you might initially think is needed - at least a foot, becasue as the yards rotate around the mast with tacking, or as you find "just the right place" for the halyard and downhaul to be attached for best performance, you'll find the extra length is needed.

Bubblehead said "And having the upper yard captured to the mast is almost required if you're going to reef the mainsail, as without it, the long length of halyard will allow a large gap between the yard and the mast on the "off" tack, no matter how much downhaul tension you use"

No almost about it. Look at how far away from the mast the yard is flying in this picture. That's the Faering Cruiser with 2 reefs in before I added a parrel.

I used a shock cord covered with wooden beads, one end tied to the yard, the other attached to a Brummel clip for extremely quick connects/disconnects (example shown is actually one of the lazyjacks). Another brummel clip was tied to the yard at the same place as the parrel. Simply grab the clip end of the parrel, wrap it around the mast and clip it to the yard. No knots to tie/untie.

Much improved.

Bubblehead is also right about all the other advantages. Do it, you won't regret it.

Laszlo

   My dory is a few years old (2015). Those were the days when men were men, and John Harris didn't spec the boat with any of those sissy reef points! I enjoy having the very simple setup. Pull the top spar all the way up and the uphaul holds it snugly against the mast. Push down on the boom as hard as possible and cleat the downhaul with all the tension a septuagenarian can give it without pulleys. That seems to do the trick. The top spar stays against the mast and the boom is as close to the mast as necessary for good sailing. Get off the lake when it starts gusting to 25!

   I've just used a tied loop of Sta-Set X (halyard cordage) as a restraint on my lug's yard.  I really haven't taken the time to make a nice spliced loop yet, but I should.  Yes, big help on lowering the sail and keeping it from going in the drink.  Also, yes, a big help in reefing.  

And yes, chances are you need more luff tension.  I made up a 3 part tackle on my downhaul and honk down on it until it's two-blocked for the full hoist. Hoist the sail to the top w/ the downhaul eased, cleat it, then fully tension the downhaul.  And I really need to rearrange it so I can get a bit more.  I'm going to move the cleat further down the mast to get more range with the tackle. The standard setup with just a line down to the cleat was just not easy to get tension.

Agree with the comments above that holding both the yard annd boom close to the mast will improve performance and make the sail easier to handle.  I second the recommendation to read Michael Storer's info on lug rig setup.  Here are a couple of pictures which show his set up on my GIS.

He advocates using the halyard itself to hold the yard against the mast.  Instead of just tieing the halyard to the yard, you run it through a block lashed to the yard, around the mast and then tie it at the bottom of the yard.  Using this system, increasing halyard/downhaul tension holds the yard tighter against the mast.  When you lower the sil, this pressure is released and the yard slides down losely but in a controlled manner against the mast.

This picture shows the bleater/vanghaul arrangement suggested by Mik.  The bleater is loosely tied to the font of the boom around the mast and back to the front of the boom.  The vanghaul is tied to the boom aft of the mast and attaches to the boat at abeam the mast.  When the vanghaul is tensioned, the bleater it put under load and serves the dual purpose of keeping the boom from moving forward and clamping the boom against the mast.  The vanghaul itself acts as both a downhaul and boomvang.  If you need more luff tension, you slide the block on the boom forward.  When you need more leech tension, you move it aft.  I ran both the halyard and vanghaul aft so I don't have to go forward to raise/lower the sail/adjust.  To get good windward performance on a lug, you need a lot of downhaul so I suggest at least 4 to 1.

Thank you, everyone. 
 

I am definitely going ahead with parrel or some other set up like it and will post pictures when I get to a system which works for me. Thank you, Mark N for the picture of the masthead using the haulyard as a restraint. I had seen an illustration but it was no where near as clear as your picture. I also appreciate Laszlo's recommendation for Brummel clips, which I didn't know about. 
Looks like adding some mechanical advantage to the downhaul is in my future.