You could in theory make a backpack's straps so that they don't constrict the wings. Set it on a Y Harness frame rather than a double D ring setup along the back. That means the main straps run directly along the spine, in between the wings. The backpack would more than likely still prevent flight, but could at least be relatively comfortable to wear. With the right setup you could probably rig something up so that the top straps had a quick release, dropping the backpack down to the legs (Interfering with running etc at that point) but then allow the wings freedom of movement to fly off (At which point what does it matter if your legs are up against the pack). I'd make the assumption that the waist belt could probably hold it on though there would still be a risk of it slipping off, especially if it snagged etc (Such as if you glitch).
In the end it would really depend on your GM, but that kind of stuff I generally let my players pull off, even if I make them pay a little extra for it and/or jump through a few extra hoops to get it.