Right now we have a playnite plugin, a muOS/portmaster app and a on-going decky plugin for steam deck. We have plans to integrate with RetroDeck. No luck with Retroarch since it doesn't support plugins, but you can always use something like syncthing for those where we don't have integration yet purpose.

Not a plan but we wrote this up a while back as an exploration into which systems/platforms we can target. We're really hoping the community continues to step up with more plugins and integrations so we can focus on the core service.

Not thinking about you guys needing to write these integrations, but if there's interest on their sides for building it themselves?

Depends on the project, the muOS team is keen to integrate RomM directly in a future release, and the Retrodeck team is willing to partner on support.