You are hearing it from someone who has done it. My Mav 88 sports a Mossberg 500 fore end. I've never had an issue with it, and it's been wearing it for about 5 or 6 years, and hundreds and hundreds of rounds.
Bought the Mav 88 in '09. Didn't like the integrated fore end, bought a busted up non functional Mossberg 500 sans barrel and stock for $40. Disassembled the Mossberg for spare parts, and swapped the action rails and and fore end. I did not have to swap the mag tube, although the Mav 88 mag tube will not fit on a 2 piece Mossberg 500, I found out.
My Mav 88 is all factory parts, except the 2 piece fore end/ action rails, and the fore end grip itself is aftermarket. If the rain holds off, I'll be heading out this morning to go shoot it. I can take some pics and maybe video of her in action.
So, if BPU claims the integrated forearm is the only thing holding their kit from working on a Mav 88, I may have to just suck it up and pick one up.