I believe Petzl caving bags these days are made of a TPU fabric - that is a polyester or nylon base fabric coated on one or both sides with thermoplastic polyurethane. Instead of being chemically welded like a PVC bag/suit, the bags are welded by simply heating the seam to 200 degrees Celsius or so (often only one side of a fabric is coated, so you can put the hot iron on the uncoated side - otherwise the coating sticks to the iron apparently).
Theoretically (I haven't tried it) this should make it fairly easy to patch a Petzl bag by just cutting patches and welding them on? I've got a 45l bag where the bottom is starting to separate, and I've had to sew the straps back on (top tip - slide some protective webbing over the shoulder straps at the base of the back and glue in place for extra protection), but otherwise the bag is good...
Anyone tried this? Any advice/good fabric?
I've found a few sites selling fabric and patterns for pack rafts, which you can make in this method.