A possible way to do it would be to extend listttransactions so that it accepts an optional txid argument:
Code:
listtransactions [count=10] [txid]
If txid is given, don't show it, nor any transaction older than it.
(As a sidenote, maybe listtransactions, with or without txid, should accept filtering by transaction category: generate, send, receive, move.)
I'm sorry I can't propose any patch here, as I can't do any decent C++.