The general consensus seems to be to give the Wire library a miss, especially for SMbus parts (Which the LTC4151 is), I've migrated to a version simply title TWI_hack which has the parts needed (notably a way to read with NAK, and waiting for ACK's on writes).
I think I'm closer, but still not getting anything useful from the chip, I tried knocking one up without optos, but I only have LTC4151-2's, which have the split output with SDAO inverted, so I can't just tie it with SDAI and have a working bus.
Next step is to go back to the isolated one and see if I can't get something out SDAO, even if I can't sustain full communications. I've also ordered some Wii nunchuck sockets so I can play with a component more well known.