I'm not seeing any length limitations. As for does it require physical protection-(250.64(B) It would depend on the size of conductor, #8 needs to be in conduit, #6 can go in without conduit as long as it follows the surface of the building, #4 needs protection if subject to severe physical damage-(Hi-Lo type setting), and as to ty-wraping it to the feeder conduit. Someone has to make the call that it is "functionally associated" I'd make the call in my jurisdiction and probably say "No".
It needs to go to the "nearest" electrode. I would double check to see if the steel is actually part of the electrode system before actually going there. Would not be cool it it were not. As you would end up with a floating neutral and a kinda live piece of building steel, or being dependant on any ground that may have come over with the feed.
If the nearest electrode were a main panel, and you were feeding from it, I don't see why the GEC could not be run IN the feed conduit, as long as it were sized right for the secondary. And it is an exception to at least the 2002 NEC in 250.30 (4) to come from the main. Its the best place anyway, reguardless of "nearest" IMO.
Mark Heller "Well - I oughta....." -Jackie Gleason