DocEd and Crowmag,
Rear grip stocks are not a problem as they have full recoil lug support, not just the outer edges like a center grip stock.
What I would do is convert the receiver to single shot by fitting a plug into the magazine window if you happen to have a repeating XP-100R model and then solid fill bed the stock under the receiver behind the recoil lug.
Generally the pistol grips on the rear grip stocks is not a real concern. The forces on a rear grip stock are different at the grip. On a center grip, it wants to rip the barreled receiver right off the top of the grip because that is where the handgun rotates under recoil energy.
With a center grip, not so much the case, more a downward force then an up and back like the center grip.
I would leave the trigger hollow as small as possible though just to add asmuch strength to the stock as possible in this area.
Kirby Allen(50)