In terms of how much freedom you have about changing the internal, I'd check how long ago they were changed.  "Internal" APIs that have been stable for many versions tend to have illicit external uses -- but internal APIs that were introduced recently (e.g. in 3.2) are usually safe to use -- nobody is going to make too much of a stink if you break their code.

As for saving f_globals, if you're going to save an extra pointer anyways, why not just save the frame pointer?