I am afraid that when this probe was developed we only had internal use of the callbacks in mind. Hence, no "user friendly" documentation of the callbacks exist. It might also be that we need to make some modifications in order to make these callbacks useful from SDP.
But if you haven't given up completely yet - the password you want to use is the encrypted password you find in the probe cfg file.
We have looked a little bit further into this issue. I am afraid that the probe callbacks currently does not return data in the format required by EC and SDP.
I'll encourage you to file a feature request in our Support system for "Enhance cisco_ucm probe callbacks with data formatting supported by SDP". I will follow up this as an internal matter but if you file a feature request you will be notified when a new release including this enhancement has become available.