I've done something like this before, but it still doesn't work as well. What I did was try two methods, one was use the impulse field (forgot the real name) of the physical object and spawned a bunch of them to create continuous gravity around a point. Unfortunatly you can't have too big of a space where the gravity affects or it'll lag from too many physical zones. If you use a trigger, you won't be able to make the impulses as constant so it'll be a little wrong. The cool thing was that with both methods, I was able to achieve orbit around the middle point for a few times. Drag from the player datablock slowed the player down though.