I see there that the actual IMAP sync is done in the client, with the Mailspring ID relegated only to extra 'features' like spying on recipients. So it might be possible to fork it and just pull out the Mailspring ID stuff? It's a big codebase though, so much depends.

Not something I'd be interested in as I don't need another Electron app (I think it is?) on my machine, but if someone had a particular liking for this it might be doable.