I'm scratching my head here - what am I missing? Why do you want to mess with your rear master right away?
Just install the longer shocks, give the belt some slack and take the bike for a ride. Then look at the outside of the belt for any signs of contact with the bellcrank - there's a good chance you won't find any, in which case there's no point lowering the rear master, right?
My belt is 6+ years old with 6,500 miles on it and my bike has had 15" shocks from new. If there'd been regular contact with the bell crank - or any other part - I'd know by now. But there's nothing, except perhaps a very fine line on the back of the belt from pushing the bike out of the garage with no load on it. But you'll have to look very closely to see it.
Me thinks you worry too much.