I have looked at one of the many displays on YT, and I'm not convinced there is any negative g. Being upside down doesn't mean you have to be zero or negative. Apart from early FW Aerobatics, I spent some time in 2001 with the Eurocopter Flight Test Dept pilots looking at the EC155, and a couple of manouevres they demonstrated, and I subsequently flew, were a) slow down to less than 40 KIAS in level flight, then roll inverted and pull through, and b) bunt 90 degrees nose down, then roll 180 degrees and pull through. Neither was anything less than positive g throughout, although short periods of less than 1 g.