Seriously, your main concern about able-bodied trans people using the disabled toilet is that it is an indignity for the trans person?
Why are you not outraged as I am that accessible facilities which disabled people campaigned for for decades are being handed over for use by some able-bodied people, like some kind of consolation prize?
Surely a trans person who had any respect for marginalised minorities like people with disabilities, if faced with the prospect of using the toilet designated for their sex OR using the toilet designated for people with disabilities who physically need an adapted toilet, would conclude that on balance, the physical needs of disabled people are more important than gendered preferences, and they would show their respect for disabled people's rights by not using the accessible toilet.
And by pointing out to HR that disabled people's rights and spaces are not to be handed out to able-bodied people, for any reason!
There is nothing physically preventing trans people from using the toilet designated for their sex, which is not the case with disabled people who physically need the toilets designated for them.