Ferns would look amazing in hollow stumps, but I'm not sure about a tree? Some trees will definitely grow in one though; you see birch and rowan growing out of some pretty inhospitable cracks. As the tree grows the roots will either push apart the stump or grow over it, as they look for fresh soil.
There's a clump of zantedeschia growing out of an old hollow stump in a local park, have a look here on streetview -
Clematis - yes, that would look great
Don't think I'd use a tree either Tetley. I'd agree with ferns. The trick is to try and make it all look vaguely natural - not usually very easy though.
If you simply want it as a nice planting opportunity then go for it - the world is your oyster!
The only reason I'm struggling with a tree, is I think it might be a bit macabre. Or as Fairy says, too much resemble a plant pot rather than a natural feature.
I love sempervivums in stumps. The chains of little plants dropping down the sides are beyond pretty. Even a small stump will do, they need so little.
I can't help thinking that a tree perched on that tall stump would be quite liable to flop over in high winds.