Well, if you take the oil pan off ....to replace, then you might as well cut out the rust area and weld in a patch. Have you called a few salvage yards??
Welding a patch on-car: I have had a couple guys share stories that they've brazed and/or welded a patch onto an oil pan before and didn't start a fire inside the engine. Including a buddy of mine who did this a couple years ago. Of course they drained the oil first. I've always been to concerned I'd start a fire, so I haven't done it.
If I *was* to do an on-car repair, what I would personally do (I'm not recommending this...just stating an idea), is pull the drain plug and put a hose from a CO2 tank up inside the drain hole. Then flood the area with CO2 (which is denser). Then weld the patch on. But again, that's theory, I haven't done it.