Morloth, I know you can't tell.. but you also can't live a superduper impeccably polite life and actually get on and do anything yourself.
Hidden disability sucks because it's hidden - because others can't reasonably be expected to make allowances for something they don't realise exists.
I go on the bus most days now that my ds is older and as I said, have taken to using the sling because I was (totally honestly!) wracked with guilt about mums with young babies who were left standing in the rain and because once, a little old lady with a walker was standing at the bus stop and I didn't see her until it was too late and she missed out because I was there with my clunky pram. I never took my pram out again..
Honestly..
However, if you are serious about considering 'hidden disability' then you have to assume every other person on the bus is more deserving of a seat than you and/or your NT dc and, practically speaking, you would never get on a bus again.
At the moment, my ds is titchy and fits into is sling nicely but it won't be long before he is too heavy for the sling and too young to be out of a fold-up pushchair so I'll just have to use the buggy space again, won't I? At least until I pass my driving test.
And I do worry that I am actually placing my ds at risk in the sling on the bus - if I fell getting off the bus, or if the bus braked suddenly, there's a good chance I could crack his head/little back off one of the poles, isn't there?
It's all grey areas and in fact, the more we discuss this the more I think yes, I would have moved for the pregnant OP but only because I have severe good girlitis. This is Not A Good Thing, either.