@periodically_pedantic I think you are right: making a secret activity pub server could work to try to scrap the fediverse at the condition of being federated with the targets.
But maybe a server + a bot like that would be quickly visible on the log of small instances and be quickly flagged as a scrapper bot (just because it takes ressources for nothing).
I'll probably search Fediblock discussions to see if such a bot/server already was seen and blocked for this specific reason.