I haven't flown any attack helicopter, but I seem to recall an Apache pilot telling me that negative g pushover of as much as -2g was used to terrain follow when the terrain was too steep for collective climbs/descents.
A far cry from flying the trimmed negative g that Nick mentioned, but aerodynamically, if you can reach -2g upright you should be able to maintain -1g inverted. Of course, the -2g period would be a constantly changing pitch attitude, so the inverted case would be of a limited duration.
Matthew.