They seem a great idea and I hope it's rolled out.
I have a serious spinal condition but is total invisible unless you see me walking. I take a lot of morphine and other painkillers every day so that I can have some semblance of a normal life and as I am also reasonably young, slim and wear nice clothes and make-up I apparently don't really tick most people's boxes of what a disabled person should look like.
It's actually quite dangerous for me to stand on public transport - not just increased pain, but if it stopped suddenly I could be in a lot of trouble and a lot of extra pain.
I try not to use the priority seats as I've had several occasions when I've been shouted at for sitting in a disabled seat when I "don't need it". Even when I've explained that I do, I've had people scoff and tell me I'm a liar. I did find getting the bottle of oramorph out of my bag and asking why I would have this if I wasn't actually got an apology, but I shouldn't have to do that.
My husband asks for seats for me, but I'm too shy and too wary of a) people accusing me of lying or b) asking someone else with an invisible disability to move so I never do. Just try and wedge myself into a corner instead. I do ask for my mother who is very severely disabled with MS and don't have any qualms about it.
I've considered buying a stick just as an indicator even though I can get around without one. Interesting to know that even with one people still ignore you.
It is appalling how people with disabilities are treated especially if it isn't super obvious - I have friends with blue badges who are accused of conning the system. One friend was told that she couldn't be disabled because 'look at you, you're wearing lipstick'! She hadn't even got as far as getting out of the car to retrieve her crutches from the back seat!
YADNBU and I really feel for you.