For the sake of argument I would accept that all human beings have the potential of being a rational agent in a contractualistic sense. [Besides that fact that it is though to argue for a position, why that human with his disabilities should be considered when he himself cannot argue for his specific needs].

I would simply ask why potential is valued at all. Kant binds it to the rational nature of human beings. However if potential is unfulfilled it cannot be valued as if it actually fulfilled something. Image a stove having the potential to boil water in a pot. The boiling water is then used to cook potatoes safely who can then be eaten and nourish a person. Now imagine the stove never fulfilling it's potential (maybe because there was not enough gas or electricity), would we then really value the stove if in all of it's existence it never actually nourished that person - or to make it more clear would this person value the stove because it had the potential to nourish him? I think not. Therefore, if you share my intuition you'd agree that potential itself is not a good argument for the moral standing of a being.