I like the burning idea. All you would have to do is include the original protosharesd daemon with the DPoS daemon, which only kicks in when you have an active delegate, to enable tracking of both blockchains. This would allow to start a transition to DPoS without a snapshot. There could also be a hard cut-off snapshot where all unburned PTS are automatically sharedropped.
You could also make a condition where, if there hasn't been a block for 7 days, then the snapshot is automatically taken (with all pending transactions included).