Unfortunately, it seems like there may be a compatibility issue with SleepTimer and the HP TouchPad. I have received a few complaints about it not working properly. While not all of these issue are reproducible, the number of complaints is concerning. As a precautionary measure, I have removed it form the app catalog until a fully compatible update is released. Follow @neathings on twitter to make sure you don’t miss it!
So it turns out the method used by SleepTimer will only stop music on the TouchPad if the app playing the music is one that runs in the phone emulator. Unfortunately, this means SleepTimer no longer works with newer versions of webOS, and will remain out of the app catalog indefinitely, unless something changes.