I found a Griffin PowerMate in the back of my drawer. They haven't been manufactured for a couple of years, and the only references to drivers that I could find say that they are only partially-supported under Mountain Lion.) Is there any hope for getting this working again via Keyboard Maestro?
Try the drivers on the Griffin site. I'm on Mojave and mine still works most of the time. Sometimes it just stops working until you restart the machine.
You might want to plug it in and see if KM sees it as a USB trigger.
You might also consider USB Overdrive (it even recognizes my VERY extinct Art•Lebedev keyboard)
I'm on Mojave with a Griffin PowerMate USB using software v2.1 (2007), and it mostly still works like new and is recognized by KM 9.x -- but only the Button action; the knob-spin isn't seen. That's okay, though, because the PowerMate Prefs Pane still lets you 'Send Key' (amongst other things), which can be handed off to KM.
What no longer works is LED dimming/mapping to Volume level
The only major issue is if the PowerMate is connected with USB, it prevents automatic Sleep; you can Sleep the Mac manually, but if it awakes for PowerNap or LAN, it won't go back to Sleep. My solution is an $8 Sabrent Switched USB 2.0 Hub; I just have to try to remember to switch it off.
I forgot to add that the v3.1 software actually works well enough, and can easily trigger AppleScripts with very fine control, but there's no standard way to easily hide/show it; it has to be launched as an app, then its editor window manually closed; this can be dealt with using KM, as well.
This version has the advantage of being able to set the LED level with AppleScript; however, it will only recognize one PowerMate at a time.
Of course, 2,1 is 32bit, so I will have to learn how to live with v3.1, which, thankfully, is 64bit. Will it work? Don't have a Catalina or Big Sur installation with which to test, sadly.
Another robust option is ControllerMate, despite it (seemingly) not being developed further (into 64bit)…