I do not think that Brompton has any legal ownership on it.

Other manufacturers have a Brompton front block mount on their bike: Bike Friday, Tyrell, Seattle Cycle...

Just adding a front block mount on the head tube of the Birdy and mounting a standard Brompton block has some limitations on the type of bag usable.

With the sport 21 degree stem, only low (260mm) frame bags can be used. A high frame (300mm) will hit the stem.

The above showed adapter changes the angle of the block wrt. the head tube and place the block a little further away from the stem to avoid this limitation.