Yes, there are existing Lemmy bots doing useful things for participants. For example, the folks on mtgzone.com have a bot that responds to specially-tagged comments and provides useful links specific to their topic.
I started building a wrapper around the official Rust API today. But it's a long way away from anything productive. Mostly just a testbed right now.
I'll post something up in a few places when I make a repo for it and ask for contributions. Want to do a fair amount of work myself before asking of anyone else.