I don't think that limiting the up travel will be necessary. The piston does not reach the top of the shock body and the bladder should compress into the cap. It is a good idea to make a very small (tiny) hole in the shock cap to allow air to escape from behind the bladder since a bladder that small will have a difficult time compensation for the stroke of the rear shock. Back in the day the Kyosho shocks were the gold standard for smoothness and they had bladders. I am interested in trying this mod too.
"I have no idea what you're talking about, so here's a bunny with a pancake on it's head."
In mind and body, we're all heading towards old age. It is slower to go by bike.