F</opt/rt4/local/lib/RT/CustomFieldValues/> (or equivalent if you
installed RT into someplace other than F</opt/rt4>). To get a sense of
the code that you'll need to write, take a look at the code in
-L</opt/rt4/lib/RT/CustomFieldValues/Groups.pm> for a simple example
+F</opt/rt4/lib/RT/CustomFieldValues/Groups.pm> for a simple example
which just uses RT's API to pull in a list of RT's groups.
Running C<perldoc /opt/rt4/lib/RT/CustomFieldValues/External.pm> will