You wouldn't be able to make a perfectly functioning physics loop without doing stuff a lot more complicated than ~99+% of the community is capable of doing. However, as mentioned above you could create a convincing looking loop in the geometry, and there's a few different techniques you could do to make it "functional", such as to script movement through it similar to a zoom tube (which is generally how Generations would do it, though in Generations it's more convincing), or to flip the gravity upside down on the upper half of the loop.
It would seem as though in regards to physics, perfect loops are getting figured out for the 2.3 update. However, there's still no perfect solution to the multiplayer issue of seeing other players 2D sprites going on the loop from certain angles.