However, there seems to be no way to access environment variables that
are already set on the execution node. It looks like the only env
variables available on the execution node are those set in the
submission script?
This is correct in Condor version 6.6. In the development series (6.7),
there's a setting in the config file that lets you add stuff to the
environment that gets inherited by user jobs. It's called
STARTER_JOB_ENVIRONMENT.