I've been reading posts on LRBHOs, and it seems that most of them operate on a push from the follower (AKA "always disengaged") . What if the LRBHO was "always engaged", unless a round was present? The idea being that with a magazine loaded, the shells in the mag hold the LRBHO disengaged, so that when a shell is not present in the magazine, or the magazine is removed the LRBHO would engage. the rudimentary design would essentially have 3 elements:
1) A reversal of the direction of force from the spring in the current BHO designs, so that the spring pushes the BHO into the holding position