The problem with the C-mags are that they generally don't work very well and with heavier rounds and pre-plastic magazines are extremely heavy with the bottom feed, so would create major problems. The top feed is to take advantage of the layout of the normal belt feed system (the PK machine gun for instance uses an adapted inverted AK-47 mechanism as the magazine feed is from the bottom) while avoiding the issue with gravity for the heavy metal drum mags and the catch mechanism. So you could probably invert the BAR mechanism to get it to work from the top (I think the Mle24/29 did that, as they basically ripped off the BAR mechanism to make it).
I'd say your best bet would be to have the BLMR dual mag system to have a quick swap out.