Finally got round to looking at this. After some effort the solution is really simple.
The Nasa Casa alexa cloud now exposes binary sensors. all you have to is ensure the binary senor you want to expose to the alexa routines is in the correct device class ( I use motion).
I have a number of leak sensors , which are in the device class moisture. The alexa app sees these devices , but cannot trigger routines with them ( which would really useful for alexa to scream danger danger ! when activated)
so you simply change the device_class to motion using the customisation’s option in the configuration menu. Select the entity, pick an attribute to override, select other :-
attribute name device_class attribute value motion