> ...and actually, that's not a problem if the defender is online, because
> they can just broadcast the highest sequence numbered tx, which blocks
> further broadcasts by the attacker.


Good point - transactions can be ordered by highest version seen before
they're signature checked. Even without that improvement it's still rather
tricky to win the race though.

I'm intending on making a prototype for myself at some point soon, probably
in bitcoinj. I've been making notes and writing some initial code - I did
successfully replace a transation on my own little testnet, then I figured
I'd submit the patch so it's easier for others to play with it. But I
haven't got the whole thing working end to end yet.