Roger:

If you look through the photo galleries for both tne regular and nesting Eastport Pram, you'll see that this is one of those cases of "different ships, different long splices" as the old sailors' adage goes.  The standing lug rig does, by its nature, present the issue of how to control the tendency for the boom to rise and the top of the sail to twist off when the sheet is eased.  A boom vang would, as you suggest, solve the twist issue regardless of how the sheet is rigged, but that might pressent some problems by putting a more pressure on the boom jaws against the mast, hard to say, plus it becomes one more thing to have to fiddle with in an otherwise simple rig.

I can see from the photos that John Harris seems to prefer the mid-boom, single part sheet held in the hands...and that he exhibits great arm strength in being able to keep the sail drawing well.  I'm not sure I'd want to try to emulate that myself in a stiff breeze for very long.

A single part sheet at the end of the boom and led down to a small block ridding on a rope bridle (acting as a traveller) at the transom might be the simplist thing for you to try first.  That would also let you grip the sheet with your tiller hand to free up the other to scratch your nose or push your sunglasses up.

Current photo #5 in the regular Eastport Pram gallery shows a slightly more complicated arrangement where the sheet leads from a bridle at the transom, up to a single block near the clew end of the boom, along the boom to another block about midway, and thence to the sailor's hand.  This would give a bit more mechanical advantage and less tendency for the boom to rise with the sheet eased.

With a standing lug like this, anything you can do to control sail twist with the sheet eased will greatly improve the sail's drive when reaching.

.....Michael