Hey Steffen,
The ClassAd is the same as what you would see from doing condor_status -long slotX@myhost > myclassad.txt
I seem to recall rooster calls it with a full ClassAd by default, but when I was trying to fake rooster calls for a little wake up utility I narrowed it down to this to wake up a random machine (I just load the data from a file):
printf "MyAddress = \"<${the_ip}:9618>\"\nHardwareAddress = \"${hwaddr}\"\nSubnetMask = \"${subnet}\"\n" | condor_power -i
I use that in a script to keep our computer labs on in the daytime, with Condor shutting the machine down if unused in the evenings.
You could probably use the htcondor2 Python bindings to parse the ClassAd, and one of the Python IPMI libraries to wrap this whole thing up into a nice replacement.