Tough question with two answers:
Yes - it is possible, but you would need an enterprise-type installation vs what comes on a standard *nix box to handle the sheer magnitude of base users (domain associations - 15k+) and very serious security and admin tools with possibly some custom scripting if you wanted to tie it into your existing management system.
You would not be able to manage it with stock mysql and phpMyAdmin. What a remote_hosts file that would be to maintain!
No - it creates a single-point-of-failure for 15k+ domains without substantial resource investments (RAID's, fallovers, replication schemes, etc.).
It also creates a greater security risk by having to expose mysql ports to the internet. Many would argue that there is no safe exposed mysql port.
Our solution was to offer a bigger/faster/better servers for heavy DB users who didn't want a dedicated server. The cost benefit analysis we did said that such a high percentage of users had such small mysql "footprints" that the actual resource consumption was negligible, far outweighing the necessary investment once you factored in utilization and overhead.
However, that being said, we do have customers that have dedicated database servers - notice plural server - because of their data requirements.
BTW - 15k users is impressive. Nice job!!