Including Planescape in a Sandbox/Megadungeon Campaign

You don't have to be high level to wander the planes - Planescape has done an excellent job in covering that topic over the years.

That said, it sounds like a good idea to me ... but I doubt you'll get many objections from the PS boards. The planes are not only a great place for PCs to move into later in their careers, they are almost a classic step in the evolution of the DnD hero. It should be quite an interesting experience for your characters to come out of their dungeon delve feeling all mighty, only to end up on the planes where for all their strength they are just another pack of Clueless sods learning the ropes. I often liken PS adventurers to Greek Heroes with my players - once you hit the planes the scope of your adventures becomes so vast that the goings on of a single Prime plane seem insignificant by comparison. Hell, my 6th level group right now has fought primordial evils hidden in the depths of the Beastlands, been manipulated into aiding the fall of an entire continent to an invading daemonic force, brokered deals over the repair of an astral conduit and the fate of the souls that spilled out of it on their way to the afterlife, unmade an artifact, posed as a god and hobnobbed with dieties (on behalf of a trickster god), gained the wrath of Gruumsh One Eye, and are currently playing a part in the possible resurrection of Gith herself.

The planes are the place to go once a group has outgrown the basic challenges of its own Prime. (or just a great place to start a game in even) So, yeah, I totally encourage it.