Here's an idea I've been sitting on for a while, in case someone wants to steal it. I have far too many side projects already.
Basically, at least on Mastodon, you can create a post that's visible to "mentioned people only", tag it with a "MutualsOnly" hashtag, and not mention anyone, and only you would see it.
I had a few ideas on how an app you'd log into would show you these posts, but they all have different drawbacks, and it might just not be worth my time. Maybe we'll get mutuals-only posts soon!
If not, feel free to run with this, see if you can make something.