It occurred to me just now that if you don't want Hold+SomeButton to duplicate the same action assigned to SomeButton, in the case of toggles you can just add it again.
So in theory, this...
Code:
Click Reaper 40364 //toggle metronome
Hold+Click Reaper 40363 //Show metronome settings - problem here is that if your click was on before you pressed and held you just turned it off
Should be remedied by this, because your essentially toggling it twice...
Code:
Click Reaper 40364 //toggle metronome
Hold+Click Reaper 40363 //Show metronome settings
Hold+Click Reaper 40364 //toggle metronome the second time