To be clear, the ring sig I suggest here isn’t about the custody of funds, but as a way to announce who is operating the mint without actually revealing specifically who is operating the mint. This could allow mints a new position in the privacy-transparency tradeoff. I don’t believe this has been implemented anywhere, all mints are either privately operated (and may steal your funds) or transparently operated (and the government may steal your funds in a seizure). 

 Neither are they - they act as a front end for any fedimint. The fedimints that show up are ones recommended by your WoT. They could modify their recommendation to show “one of these people proved they run this mint” though