OK thanks guys, maybe I'll go for a simplified version of the 3 in 1 tack/halyard/snuff combo after all (I'll spare you the diagram Andrew). I'll see if I can check out some other real life setups too. We have a Tiger at our club and some Nacra F18s too. Together with the help you gave me that should give me a good idea of the type of blocks I need and where to put them.
I like the filed off swivel trick, that sounds like a good and simple solution for me.

Thanks again everyone!