I did a small addition in my entry way of my house and i didn't think about insulating the floor until after it was finished. There really is no access below Its only like 5x10, so its not a huge deal, but you can definitely feel that its cold right there. The floor is tile and im not ripping it up. The only access is in between the 2x10 floor joist above the stem wall on the original foundation. I can about poke my head in there, but not enough room to do any good. I doubt theres any way to insulate it, but i thought id check if the pros have some kind of miracle method.
You could probably have a foam company come in and insulate it. They have some long wands they could stick in and spray the underside of the floor. It won't be cheap but probably would be worth it in the long run if you were planning on staying there. Of course the comfort is an additional consideration.