Whole lot of variables. How was it determined it is rusting through, and from the inside or the outside? I would think it would be very unusual from the inside? If it is leaking, maybe a seal or gasket joint?
1. If the pan is rusting through from the outside, you may be able to patch it from the outside, if you can get to it.
2. Maybe the engine could be separated from the transmission and blocked up enough to drop the pan, head room permitting?
3. If not, you might consider having it rebuilt while out, depending on the amount of hours on it? Or maybe just leave it alone?
Interesting situation, let us know how it comes out