You can get a left side drive crank for tandems. A fixed hub would tend to put a lot of extra stress on the lockring, but not more-so than the chronic skidders do. LHD is actually relatively common on BMX bikes, so it's easy to get a LHD ss hub and freewheel. In fact I can lay hands on a LHD BMX wheel with a freewheel on it. PM me if you'd like to trade the hub and FW from some random bike bits.