Adding 16 bytes to every in-memory transaction to support a feature that will not be used by 99.9% of users seems like the wrong thing to do.

How about instead keeping a map<transaction_id, pair<priorityDelta,feeDelta> >  in the memory pool class, and have prioritisetransaction add to that map?

Finally: need to check to see if the free transaction rate limiter code needs to take this into account (I assume you should be able to prioritisetransaction <txid> and then if you receive the transaction over the network it should sail through the limiter and make it into the memory pool -- or is it assumed that the transaction will get into the memory pool some other way, like a sendrawtransaction call?).