Yes, you can!
The magic is in the System Actions plugin. Add it to your Rackspace (the Global Rackspace if you want it to be universal) and you can then map widgets to the System Action parameters “10-Metronome” for turning the metronome on/off and “2-PlayStop”. Both of these need to be on for the metronome to work.
Seems @edm11 beat me to it.
To add to this, look at this previous post:
[Gig] Custom metronome with visual feedback