Meaning no disrespect, I just have to say the following...
Idler pulley, yes. Inside the belt....never seen it done that way.
It will actually try to lift the belt off the pulleys. You need the
idler-pulley and pivot to be outside the drive belt path. On my homemade
minibikes, I used a rope attached to a foot pedal to pull the idler-pulley
down, tensioning the belt for driving. Release the foot pedal and you
are back in neutral. Worked every time. I used screen door spring hinges
for the pedals, rope to the idler pulley, and flex wire to the carb.
Of course, use whichever way appeals to you. As for me...
Good Luck,
Wayne