Hello Mick,  I don't know if any SWD builder frequent here, which probably explains the lack of replies. 

SWD was a finalist when I was looking at oar/sail boats, but realized that it was just a bit too long for me to be able to build it in my garage, and built a Goat Island Skiff instead.  

For what it is worth, I would not want to regularly step/unstep the mast afloat.  I have done it dock side a couple of times and it is not fun trying to control my 18'/21# mast while standing in the boat that is bobbing around a bit.  The SWD may be a bit more stable since it weighs a bit more.  The other point is that on the SWD you may also have to step the mizzen.  That will obviously be much easier than the main, but it will add to the number of lines/things in the boat to trip over.  

I'd ask CLC what they think.

   I have stepped the mast and rigged my skerry while floating, admittedly in a fairly quiet cove.  It was a very busy ramp, so I stacked the mast and rig neatly along the port side and rowed out of the way to quietly rig.  I did drop my lunch hook so I didn't go too far while paying attention to rigging.  I wouldn't do it in a busy channel or with much in the way of waves.  I'm not athletic, but in fair shape and am ok standing in the skerry in the right conditions. It does take some balance.  The SWD is more stable, but the masts are bigger, so it's a balance.

Though not featured on any smaller CLC boats to my knowledge (PocketShip is?),  a sailboat with the mast on a hinged tabernacle could be a good thing.  There are other kit designers with nice boats that do that.

I sailed a classic daysailer on the Norfolk Broads in Britain for an afternoon. Remarkably adapted boat, with a lot of canvas and quite maneuverable for tight waterways.  Many of the waterways have stone bridges w/ low overhead.  The boats feature tabernacles that the practiced crews can use to approach the bridge at speed, drop the rig back on the deck, shoot the bridge, and raise the rig on the other side.  I didn't, but it is cool.

   The Northeaster Dory is a great rowboat. The lug rig is easy to step out on the water. I've done it often. The dory is bigger and more stable than a Skerry so easier to rig afloat. The mast and spars all fit in the hull with just enough room for working the oars. Take a close look at this design.

Likewise, the lug rig on a Passagemaker Dinghy is pretty manageable that way.  No need to stand up fully, and the beamy girl (does this make my boat look fat?) is pretty stable if you don't get too far forward.  That's a lot smaller boat than the Southwester Dory you're contemplating, plus I don't know that I'd want a motor along in a Passagemaker Dinghy if I meant to also sail, as the Southwester Dory is meant to do, a true three-way boat.

Still, if the Southwester Dory is stable enough that one might safely stand just aft of the main (fore) mast partners, I don't think it's unreasonable to expect to step the mast.  I think the spars are all short enough to fit inside of the boat and still leave room to row effectively.  I used to do this often with a Sea Pearl 21, a bigger, heavier cat ketch with masts nearly as long as the boat.  It was all about being able to swing the masts up into position quickly and then drop them into the steps in one smooth without trying to wave them about like oversized flagstaffs.  Given the ability to stand on the bottom right by the mast partners, I should think the Southwester Dory would, if anything, be much easier, given her shorter, lighter rig.

.....Michael

   Thanks all for your thoughts. I will have a look at the Northeaster Dory as well

   Is there any reason you cannot step the mast before rowing?  A gridge, perhaps?  If not, step the mast at the dock, and furl your sail on the mast.  This is how I do it on my cat ketch.  (The mizzen has been moved forward to a secondary position.

   I meant "bridge".