Thinking it over, it would be very easy to do this actually.
Not saying it's a good idea, just that it's technically possible - and made easier by the technical details of federation.
A modified, rogue instance could scan each post/comment/toot as it federates in against a list of keywords, and use a bot account to report the ones that fail the scan back to the original instance.
Worse, this rogue instance might not mark the account as a bot.