Gyp-c2,
The quick answer is yes you can heat treat a frame and slide after using it. But that aint the whole story by a long shot. Inevitable, some warpage will happen, even if you have some fancy fixturing. Also, heat treating is not too kind on nicely finished surfaces. Your gonna have to completely redo that. While it can and has been done, it is best avoided these days. Total cost (materials and labor) to do it right start to finish probably exceeds a new frame or slide.
The best frames and slides are machined to final dimensions after heat treating. Some things are debatable, but in the world of mechanics, this isnt one of them. Your main problem is the individuality of the project. Several companys heat treat after machining, but they have developed a very particular procedure for it, and even they may have varying rates of success that they wont mention for obvious reasons.
As they say in NY fagedaboudit