I think some proxy work was also done in CVS 1.12 but I'm not sure if
it was a full read/write through proxy.
Yes. cvs 1.12.13 has a full write-through proxy. It assumes CVSup or
similar mechanism would maintain the secondary servers and that the
primary server would be used for write operations directed at the
otherwise read-only secondary.
I am not sure if anyone is actively using this feature at this time.
-- Mark