I am not sure about the finish on the SA 1911 but the Melonite finish on their XD models is unengraveable. Usually an anodized finish is on a gun with an aluminum alloy frame. I have cut through anodizing on S&W pistols with no problem.
I've seen Images of a slide with "Through the Blue" engraving done on what appeared to be a Range Officer model. I think the finish on those is more akin to a parkerized finish than a hardcoat. Roger is spot on about the XDs though. Hard as a diamond. Give Springfield a call and good luck!
Layne
At the last gun show I meet a dealer who will leaser engraving on any Tactical guns and I got his business card and I'll be sending anyone to him with a Tactical gun with a funky finish who call me. J.J.