If the BAR uses a standard M249 mechbox, than the P* will be a direct replacement. The most modification you will have to do is drill a hole or find a route for the airline to go through.
As of right now, there is no way of saving 2 modes on the FCU, but what you CAN do is run 2 FCUs. or just open up the battery compartment and reprogram it quickly...way cheaper.
You can setup the 2 FCUs in 2 ways.
1. Control line switch
2. Power line switch
1. Control line switch will require a switch that can toggle 7 lines at once, or multiple switches. (2 for the box mag controller, 5 for the P*)
You will need to provide power to both FCUs at once and have the 2 different ROFs programmed 1 in each.
2. The power line switch will be way easier as the switch will be much easier to find. You will need to split the FCU control line to the P* though. Program FCU 1 with ROF1 and 2 with the 2nd ROF.
both of these will work perfectly fine. The only thing i can think of is that accidentally switching while shooting may not be good.