I'm pretty sure that if you were to take your rig outside, you could crank it without feedback, as long as you kept the guitar body out of the beam from the speaker. But in a small room, the guitar is going to act like a microphone. And it will 'hear' your speaker very clearly, no matter which way you face. In that tiny little reflective-walled room you guys jam in, you have nowhere to hide. I expect that it won't much matter which way you point the amp, its going to be fairly tightly coupled to the guitar. Gain adjustment isn't likely to help, because the problem is caused by the sound level. Simply put, its just too loud.

Making a box behind the pickups isn't likely to help either. The soundwaves from the speaker would still cause the guitar body to vibrate, which will cause the strings to vibrate, which will be sensed by the pickups. Thats what they do.

So, you can try to do something to the walls of your jam space to deaden reflections. Or you will have to restrict your use of the hollow-body axe to other places.