Googling found pretty much nothing, only two posts on KVR, neither with a solution. While this fixed the problem, it took loads of searching/guessing and was really frustrating.
If anyone knows how to do that please say. In the MIDI editor (either inline or dedicated) you should now see MT's own labels if you zoom in enough. Click that, navigate to the folder containing MT Power Drum Kit 2.txt that I assume MT adds when you install it (for me it's C:\Users\Pictures\AppData\Roaming\Tracktion\Waveform\MIDI Note Name Maps) and select it. 'Load MIDI note names map' is at the bottom of the menu, but there's no indication that the menu scrolls, I only noticed by accident. Click on the track header to focus it, go to the Actions menu in the Browser (not the Actions button in the bottom-left menu). You have to be using a MIDI clip, not a Step Clip. You could edit/add the names of the Step Clip channels manually, but the proper thing is to load a MIDI note names map for the VST using the track's Actions menu. The problem is that using MT Power Drum Kit 2 in a Step Clip in Tracktion Waveform doesn't show the right drum names. I found the solution which I'll write here in case it helps someone else.