Why not use two webbing loops attached by a small piece of rope?

Have the loop from the mains'l going around the boom and have the outhaul attached to the end of the sail. Then have another loop behind the first one that the Mainsheet is attached to. Have the two loops tied together by a 3mm piece of spectra, for instance, about 10-15cm long.

When the outhaul is tight for upwind, the mainsail loop holds out the mainsheet loop. When the outhaul is loose and the boom is wide, the mainsail loop can move along the boom to wherever it likes and the mainsheet loop will be further out on the boom and not pulling it out as much.

Personally I don't find it too much of a problem. When running downwind in very light weather where the mainsheet pulls the foot flat(ish) when the outhaul is loose, you're most likely moving at quite high apparent wind speed at deck level anyway and would want a flatter shape. Once the breeze fills in a little, then the sail has some force in it to pull the foot into a curve anyway so its not too much of a problem.
I think getting the amount of twist in the mainsail and especially the jib in very light weather is much more important than the outhaul trim.