That's backwards to every BP/W setup I've seen...
Potential disadvantages:
1. If you need to tighten the wing nuts, you'll have to take the tank off first
2. If the bolts have a square shoulder to stop them turning, the holes in the BP are usually round, so they'll spin and/or damage the BP
3. If you wanted to swap to double tanks for a dive, there may be more work involved
4. The wing nuts might interfere with the proper seating of the tank and/or routing of the cam bands
Not sure how you don your rig - over your head I'm guessing - but if you're worried about cutting your face, maybe you should get a BP pad/pocket... and perhaps you're doing it wrong?