There are generic aftermarket lift off hinges available but they're designed for fiberglass doors. Factory hinges are difficult to replace. One half is bolted to the body. That's the easy part. The other half is welded to the door and difficult to remove.
Third gen doors are very heavy. Even the stripped down ones. When I originally yanked my factory doors off, I was able to put them on a scale. Full power option doors, complete, were 105 pounds each. I sold these.
I picked up a set of non power option doors from a junkyard. Removed the mirrors and filled in the hole. Cut out most of the inner steel and cut out the crash bar. I left enough of the inner steel to hold the window regulator and still had the glass windows. Weight was down to 75 pounds each.
Since then I've cut out the rest of the inner steel and got rid of the regulator and glass replacing it with a fixed Lexan window. I never had them off to weigh but rough guess is around 50-60 pounds each. It's unlikely you'll get them much lighter unless to take it down to a bare outer skin with minimum framework for the latch and hinges which could be very possible.
Removable doors are usually only done with fiberglass doors. Third gen fiberglass doors are about 10 pounds each not counting hinges, latch, Lexan etc. At $370 each for new fiberglass doors, I'll live with the extra weight for now.
It's hard to describe but a removable hinge needs to be designed so that it can only be removed from the hinge when the door is fully open. Typically they're simply a pin through a tube with some sort of cotter pin holding it on.
Here are some pictures of a tube chassis t-bird to get some ideas on how to do a hinge system. The slot cut in the one hinge is for the bracket on the door. When the door is closed, the bracket fits into the slot. When the door is opened, the bracket clears the slot and allows the door to be lifted off.
http://img.photobucket.com/albums/v6...progresssm.jpg http://img.photobucket.com/albums/v6...dcagesmall.jpg http://img.photobucket.com/albums/v6...odoorsmall.jpg http://img.photobucket.com/albums/v6...hingesmall.jpg 
