Are you sure that that effect is not present in our solar system? The outer planets are harder to observe and may have many more moons than we currently know of. Especially moons of the barely-more-than-debris kind.
That stands to reason, of course, but I don't want to create stupendious numbers of small rocks flying around in orbit. It will just give Orbiter a lot more numbers to crack without giving the player anything he can orbit.
I experimented around a bit more, set the outer limit to a simple gravity-limit (0.002 Gs, a bit less than the influence on the outmost "essential" not-captured moon in our solar system), and am currently playing around with the inner limit, which was set too far out by accrete to be usable for gas giants. However, it turns out that there aren't any relevant masses formed beneath that limit anyways, so reusing the accrete algorithm to simulate the accretion around gas giants might not be such a bright idea after all. I'll have to play around with it a bit more and see what other options I can come up with...