Well I certainly feel dumb. The answer is right in the documentation, I just failed to find it (I did look first). The system-wide psqlrc, and the ~/.psqlrc files fit the bill perfectly, and the documentation explains it all quite nicely. I accomplished my main goal better than I had even hoped. Our two production servers are locked down and do not accept external connections. I wanted to customize the prompt so I could tell at a glance which I was on. The documentation even explains how to have a tcsh-like color prompt, which I use quite extensively in bash. I now have a yellow prompt on one server, and a red one on the other. How cool is that!
my system-wide psqlrc (/etc/sysconfig/pgsql/psqlrc on CentOS):