What would it take to turn a sail designed for a boom into a boomless sail? Namely... a H14 sail, or a H16 sail cut down to fit a H14 mast?

You need to attach a boomlet to the sail, so that you can attach your blocks, and have some adjustment as to where the blocks hang. I got one through Murrays, and it was not cheap. You may be able to put one together yourself.The attached pic is the only one I have that shows it at all. Good luck!

All the Hobie Waves and one of the G-cats in our club (the G-cat had a custom boomless sail made by Calvert) have a simple three hole aluminum plate riveted in place on the clew of the sail. It doesn't allow for much adjustment while under sail but it does give some opportunity for tweaking. Most of the Nacra's have the boomlet as Brian described (and while certainly safer, even that can still leave a quite an impression on your forehead ).

All the stock G-Cat sails have a few plates similar to the headboard with about 5 holes in it. I just shackle right through whichever hole would be best at the current conditions. Super-simple and maintenance free.... There does seem to be an over-large batten in the very bottom of the sail though.