I've been racking my brains (OK, I thought about it for 5 mins) on how you could make a mast extension that, instead of having its length fixed and then have a pulley to downhaul, the mast extension itself could be adjusted in situ so that your sail is always downhauled to the bottom.
That way you wouldnt have to remember what length to set your mast extension for each sail and you could always have a perfectly rigged sail.
The only thing I can think of is a screw system, but that would get clogged up.