It depends on how such an object would be handled in US3. In US2, it's just treated like any other body, but with a fancy texture.
I'm assuming you mean a full-on ring world, with its own mass just like any other object, but where something would have to collide with the ring to actually collide, and an object could pass through the center of the ring if it does not.
I don't know enough about how US3 will work from a programming perspective to be able to tell you if that would even be possible, but regardless it is a very complex object, mathematically. There are relatively simple solutions for the force of gravity at a point along the
x-axis relative to the ring, and at points on the plane of a ring or at a great distance from the ring (where it can be treated as a point mass), however to calculate the force due to a ring at
all points relative to the ring is a bit of a tricky thing, especially if one considers tidal forces. To be able to do so quickly (as in, in as few steps as possible) so as not to cause performance drops would be quite a feat for something that is meant to be run on an everyday computer. But again, that's relative to what I know of how US3 will work, which is a very, very, very small amount, if at all. For all I know, Dan could have a few tricks up his sleeve that I don't understand.
If it is possible, then I would think it should wait until an update sometime after US3's release, as it is not necessary to the development of the game as far as I know, and again, because it is such a complex object.
I think I said "ring" far too many times in all that, but that's ok I guess.