I asked the same question a couple years ago when I was doing the Kodiak door handles. According to Chevy dealer they did not make a CC HD, but I have seen them and post a few pictures too. I never did find out though if it was a body works co that did conversions. When I ran the VIN at dealer it came back as 2dr.
Monroe truck and equipment and I believe one other well known upfitter did the crew cab conversions on the C3500HD, as well as 4wd conversions on the HD. most ended up being crew cab 8ft bed, but there were a few crew cab 111ft beds. Union pacific railroad loved the crew cabs.
I dont know if either one could officially be "ordered" and have a RPO for the options or not.
There was a crew cab 11or12ft bed truck for sale near by at on point (in my search area) they wanted 8K and it wouldnt start and paint was peeling BAD.
I would imagine. I think in the eyes of the DOT, it wouldnt be and worse than a SAS. But I have no experience in the matter either, I am just thinking from a common sense stantpoint, but our government doesnt think the same way, so one would definitely need to check it out.