I'm glad that nobody else seems to know what it is, either.
I'm aware of a couple of techniques, such as rotating the left hand around the pistol grip so that the button can be pushed in with the thumb, and, of course, trying to get the trigger finger up on the "wrong" side of the trigger guard, but I really dislike breaking the "firing grip" with the left hand to disengage the safety, so a reversed button would certainly seem the best way to go, if you are stuck with a crossbolt.
I prefer the Mossberg tang safety, for lefty or righty.