Two thoughts:
1) BTER is supposedly honoring PTS for BitSharesXT; they could probably be convinced to honor PTS and MMC for LTS.
2) An idea for snapshots/airdrops that has been kicked around here is a "half-life:" make unclaimed balances decay over time. Clearly, this assumes that users must do something special to "claim" their balance beyond simply importing private keys. It doesn't totally solve the problem, but might be something worth thinking about.