As a simple solution, I can sudo symlink MY ~/.psqlrc to that directory (/var/...; changing ownership also to postgres), but there appears to be some underlying issue, as Pg should find ~/.psqlrc, correct?
What does "your" ~/.psqlrc have to do with any of this? If you are executing psql while masquarding at the postgres user its the postgres user's ~/.psqlrc file that will be looked for.
Typically one doesn't execute psql as the postgres user. They configure the system so that they can execute psql as a regular user. You should probably do that and then everything should work like you are thinking.