it seems you've done some research, there aren't more than a hand full out there, i haven't tried any of them personally

I'm dealing with a back injury myself from riding, sitting too much & a weak back in general. i have some bulging in my lower vertebrae. using my old board that didnt fit me well, i fell hard 3 times on my left ass one icy early-season day at killington, the next week i twisted ever so slightly and POP went my back.

i'm debating an ass protector. best of luck getting protected and having a safe but gnarley season!

Ive been using one from HEAD.  The model is the Thorac Unit.  It hasnt affected my mobility or agility when ive worn it and its helped with a couple bails for sure. 
Best of luck

Sometimes they can ride up, but that also depends on how the fit is on you and how tight you have it done up.  That doesnt mean it needs to be super tight, just has to be snug, easiest way to tell if it will is when you try it on (i HIGHLY recommend trying it on as proper fit is important with these) is get it snug and flex your body like you're riding and see how it moves and reacts with your body.  I havent had any trouble with mine riding up on me and as for length the proper fit should come down to just above your tailbone give or take an inch and just below that bump at the top of your neck, so it more or less stops you from hyper extending and some impact protection. 
Hope that makes sense and helps