How about this for an idea, I think i have put this forward before but here goes !!
Have the mainsheet start at the boom end, lead to a block on the transom strop then back to the boom end again.
Then lead alone or inside the boom to a turning block at the gooseneck area.
Turn 90d down to a turn block at the rear of the mast or on top off the centreboard box.
Turn 90d to lead to the thwart area and enter a centre mainsheet cleat from below somehow.
I have seen this fitted to an Albacore dinghy and it keeps the inside of the boat clear of mainsheet ropes.
Needs some working out with regards to the fittings used and the route the sheet takes but it may be an answer to the problem?