I'd be interested for an inexpensive deal, for sure.

I know this sounds weaksauce, but has anyone considered that @nihontastic should send a message to the alleged 'hacker' account and ask for the coins back? Maybe there WAS a mistake somewhere, but the guy on the other end is actually a nice guy. It can't hurt and will only cost 1 burst. @nihontastic

In case anyone is still wondering about this, current wallets include code that puts the 'payload hash' into the block's hash, which then becomes part of the block's generation signature. I believe older wallets also did this, so the issue the OP is proposing already exist in the code. Fancy, eh?