[Ann Drysdale]

A Radar key, perhaps? Though this might imply that the transgender individual is to be regarded as "disabled".

However, if these these facilities are regarded as places of privacy where those not able to use communal toilets can deal with whatever sets them apart - wheelchair, prosthesis, sexual identity, stoma - it might solve the problem.

These toilets are "unisex" but designed for the safety and dignity of those with a special need.

But you're right, Nico, it ain't easy. Especially where I live, where all the public toilets have been closed to save money.