Forgive me ... my lead engineer has rebuked me for mis-remembering this ...

It wasn't people preserving ownership and creating 0:0 files, it was people preserving permissions and sending us chmod 0000 files.

They were root on their end so their broken permissions worked but as soon as they hit their rsync.net filesystem, the 0000 permission meant they could no longer read it.

I think my original idea is still possible: you could do this on purpose (perhaps with 0400 permissions) and create write-once backups on any remote rsync host.