Thats actually a good idea. You don't even have to make it a fixed value. The receiver uses how much money was paid as a (spam) signal. The bitcoin transaction

Thats actually a good idea. You don't even have to make it a fixed value. The receiver uses how much money was paid as a (spam) signal. The bitcoin transaction can just go to /dev/null with a hash of the message as data, so don't have to pay somebody ( ;) ) and you cannot track who sent data to whom via tracking the bitcoin transactions.