I've toyed around with the idea, but scripting gravity manually, even "linear" gravity, seems to foul up the Physics Engine. I've had problems with the Physics engine and constant contact (ie standing on the ground)

In one test, I set the PE gravity to zero and added centralforces to my entity to emulate gravity. The MOVEMENT was flawless and looked good BUT upon collision with the ground, the entity seemed to have the "shakes". My explenation is that that something about the contact point(s) on the gound makes the entitys slightly unstable which would normally not be a problem, but since the PE is still applying the central force (gravity), these instabilites lead to the shakes.

I'm guessing that using more than just one force spread on an entity (instead of one central force tugging at your center of gravity) and/or adjusting the entity geometry by attaching extra, dummy entities as stabilizing blocks might help, but I don't know.

In principle, if you can resolve the constant contact problem with the ground, there is no reason why you can't build a LARGE sphere and have accurate gravity.