Regardless, editing terrain INGAME is impossible. Even 'enabling' changes are impossible. If you edit it in a mod, it's either edited (mod active) or it's not (Mod not active). It would be really cool, but it just isn't supported.
Hmmm... I haven't encountered limitations about the following, (either documented or reported), yet I know there would be some problem to work around, however the impossibility isn't so much that of an evidence : could a combination of moving statics, enable/disable exterior world objects, with some underlying mechanics, be implemented (via quest or "beacon" scripts, as well as a storyboard adapted to counter odd and contextual technical behaviors - like the PC must get out of some cell or go to an interior cell so as to begin some "building" quest enabling/disabling stuff in an exterior cell etc.) ?
BTW, I come from Oblivion (I haven't had the time to mod FO nor NV), so all that I'm writing here could sound "noobish"... Have Bethesda resolved the problem of moving statics or activators not rendering their collision data after having been moved via script? There are work-around with Oblivion (the engine treated the persistence of added objects differently given the distance from the center of the map), but they're not elegant. I wonder what's the state of this issue in Skyrim...