As it turns out the answer was that SLiM, which #! uses to control its Window Managers is using x-session-manager.
The name of the process I was looking for was not 'awesome', it was 'x-session-manager'.
So a quick:
$ wmctrl -m
Returned:
Name: awesome
Class: N/A
PID: 3091
Window manager's "showing the desktop" mode: N/A
Solving my PID troubles.