Where is the rust? Is it on the lower front corner or lower rear corner of the jam? The front corner, I feel, has some affect as a structural area. My rusty Celica has heavy duty rust in the lower front corner of the passenger door jam and the body makes lots of creakings and pops from that area.
Repair it now while it's small. More than likely the rust is even worse in the area where you can't see it, on the backside of the metal. Your best bet would be to cut a small hole, or drill, the rusty hole to a larger diameter. Treat the metal on the underside with POR15 to stop the rust and fill the with fiberglass sheet or chop.
Just a drop in the bucket.
Scott