Well there's only one way to find out in that respect. I was referring more to the overall position, rather than the angle; but I've never tried slapping something velcro on to my back and I don't think anyone else here has by the sounds of it, so we'll only know when someone tries it.
imho, I think there's probably a reason that you don't ever see people doing it, either in airsoft or all the millions of people in the militaries of the west. They use and are issued conventional backpacks which you can easily swing off and access then put back on, though I'll be the first to admit that normal packs don't always interface with PCs that great (varies a lot by pack, PC and armour). My personal feeling from trying an awful lot of kit in over the years is that I'd prefer to use a small conventional pack along the lines of a Tac Tailor removable/Grey Ghost lightweight. But each to their own ofc, be interesting to see what the end setup looks like and the idea is interesting either way.
