@evan @jcoglan @darius yes! It's e.g. notable that (at least the last time I looked) Mastodon totally lacks a plugin system (front- or back-end, much less an "ActivityPub filter proxy"); such a thing would be amazing for the sort of play and experimentation you're pointing to.

@geoffreylitt has been doing a ton of work in the direction you're pointing, in case you haven't come across his work already!