I'm not fully aware of the abilities of Tivoli, but using the command line tool you can get the status of a disk like so:
pgpwde --status --disk 0 > C:\pgpwdestatus.txt
Thatll give you a text file, maybe Tivoli can inspect that text file for a specific string? If the Highwater mark and the sector count are the same, the entire disk is encrypted.
You could also use the >> C:\allpgpwdestatus.txt command to have a central text file so its all in one place, but I'm not sure how you'd determine the difference between one computer and another, maybe doing it like > \\networkshare\%computername%.txt would be the best way