The good thing about BFG, being a 'dead' game is that the playing communities are relatively small, and usually 'the opponent's consent' is not hard to get, especially in case of internally consistent rules-sets such as BoN.
It all comes down to: "does this make sense for us to use?" and not if it's official or not. Official or not official in this case is a poor-man's-argument. In case of a game the main argument should always be- are we having fun using this?
May I remind you that BoN was in fact published on the old specialist-games website, along with official GW rules. This is probably more then the rules developed under HA will get, if GW will maintain it's current policy towards SG.