I've been trying to figure out how to add an action to devicenotifier that would perform a simple mount on a USB thumb drive (or any other removable mass storage device) without the need to open Dolphin each time (wasted time and CPU/memory resources). My search has led me to pmount-hal as a possible way to do this. So far, I'm not all too sure what pmount-hal expects and what devicenotifier provides in terms of device names, etc.

Does anyone have a good idea how this might be accomplished?

The Device Notifier is powered by Solid Predicates, which instruct it which devices match the action. KDE 4.3 included a editor for these, which includes information on url macros such as %udi.

System Settings > Advanced > Device Actions

I understand where to edit the actions, in fact that is why I'm curious about whether pmount-hal can be used by devicenotifier. Is there some sort of documentation or HOWTO about these actions?

As long as pmount-hal does not need root access, then it should be possible. The Device Actions control module does have some limited documentation on them as well. Otherwise you will need to ask in #solid on FreeNode.