...you do know that the FWDs diff is inside/part of the transmision don't you? That being the case, it's no surprise to me that no FWD Volvo owner would want to pay the labor to have their trans removed and dissassembled for such a mod as it'd be very costly.
If the car was still under warranty, adding that part could void the warranty being that the trans didn't come that way from the factory (in the US) and thus if and when anything else related to internal trans parts failed, the customer would be out (IE:no coverage).
I can see adding the RWD MVP unit to a RWD Volvo (not multi-link however) as it's fairly easy to do and even if it didn't hold up, one can find a good used rear axle assembly for those (700s/940s) fairly cheap. The FWD trans diff is another story in itself and not anything that an amateur DIYer is likely to do himself IMHO, at least not your "average guy" with basic home tools.