I have to agree with idroba, I don't think rivnuts are the answer. Rivnuts are good for some things but in a strength application, especially one subject to the stresses those hinges go through they wouldn't stand up to the punishment.
I've swapped several doors around and I would be hesitant to use anything but what is currently in place.
You are right, it's going to be a royal pain to gain access to the nuts inside the wall. My best suggestion is to buy one of those stepped drill bits
and drill holes centered over each of the bolts. Even with this method you have to be careful in the length of bit that you use. Also you have to drill part of the door frame flange in order to get inline with the bolt. The hole needs to be big enough to easily get a deepwell socket into.
The nuts are covered with vulkem so you have to dig that out of the way. The biggest problem I had was the nuts were so close to the edge of the door frame I couldn't get a socket on the nut so I had to enlarge the opening enough to get a wrench on the nut and then I used a really big screw driver from the outside. It was NOT a fun task.
If you ask me the way they should have been fabricated is with a captured nut plate on the inside of the wall. This would allow the door to be removed without worry of the nuts falling inside the wall.
Sorry there isn't an easy anser to this one