So now that Microsoft is screwing Minecraft over with the report feature and all I got reminded of something I used to use back when I didn't have Minecraft Premium.
https://mineshafter.info/
It's essentially an alternative login/identity/whatever server for minecraft. It lets you have a skin without premium only able to be seen by the Mineshafter userbase.
it uses its own launcher, servers can be supporting Mineshafter specifically in some way, but even on non-Mineshafter servers if 2 Mineshafter users encounter each other they will be able to see each other's skin.
Though it's quite old and it seems like it hasn't been updated in a while. But the concept of a third party Minecraft identity server is there
well reading about it again it isn't exactly what I described lol, but I am pretty sure what I said is possible to get done.