Since access to /dev/ipmi0 is restricted to root, my kludge was to create a root cron job which runs every N minutes, writing the appropriate value to a log file:
ipmitool -c sdr get "Inlet Temp" | cut -f2 -d, > $LOGFILE
Then, the Ganglia python module reads that file. I followed the example.py module which is distributed with Ganglia, and the associated example.pyconf.
The code is at my github repo.