Could this be solved using a Wrapped REP token that just follows an oracle price feed for REP? This way even if it does fork, you still have the price feed and status of the new REP token. The wrapper can do something fancy like verify the redeem-ability/convertibility of the token in the wrapper.
Are there time limits to converting old rep to new rep? Old rep is only worthless if the holder is unwilling to convert it to the new REP, so can’t we just assume that anyone holding oldrep can convert to newrep?