Most collision shops probably aren't going to want to mess with it, honestly. Atleast that's the way it is in my area. How bad is the spot in question? What does the rest of the frame look like? Perhaps if its just a small spot that could be cut out and have a good patch welded in on either side of the rail, a shop might do it without giving you much hassle. If it needs an entire section of the rail replaced, that's most likely where they will give you trouble over it. Ive seen some pretty shoddy frame patch attempts over the years when checking a vehicle over for state inspection, and Ive failed every one of them. Way I see it is, if I put a sticker on something that has structural integrity issues such as a frame rail, god forbid, something happens and it breaks, you possibly have an accident, and possibly kill yourself and/or someone else. Johnny Law will come a knockin', asking me why the vehicle was on the road, and my ass as well as the shop's collective ass gets fried, and Im not ok with that. A quality patch on a still decently solid frame I'd pass with no issues. I guess it all comes down to what it needs and what the rest of your frame looks like. Without actually seeing it, I would have to question the rest of the frame if its already got rust that serious. Just my 2 cents.