I will give munin a try, however I would prefer nagios since I use it to monitor my whole network (which is pretty big).
I've only just this minute looked at Naigos (used Munin before, however) but it looks to me as if you could fairly easily hack comboy's script. If I read the documentation
correctly, Naigos wants a plugin that simply spits out something like "GPU Temperature: 75.4". You could do that with comboy's script by removing everything except
the else-block. (And maybe a bit of ruby-hackery to pretty-print the output for Nagios/human-consumption).
...but I know nothing about Nagios, YMMV, IANAL, consult a qualified physician before commencing exercise, do not taunt Happy Fun Ball, etc.