The feasibility studies of Cassini's trajectory from cruise through orbital insertion were explored in-depth on an Apple II+ computer in the early 1980's. This was a 1MHz machine with typically 48K ram, and Microsoft BASIC.
As it stands now, its not a matter of Orbiter being able to control a rocket. The architecture of Orbiter isn't geared toward that outcome. While Orbiter obeys and "understands" the laws & equations that govern a real rocket's trajectory, it is "wired" to do other things like drive a visualization engine. The framework is not a good fit, not even a fit at all. It would be like like sticking wheels on a boat instead of a propeller. The physics behave as you'd expect, but the coupling to the real world is all wrong. So you see, the question isn't really a fair one.
What Orbiter can do is demonstrate the power of today's microprocessors. And today's chips (even ones from 20 years ago) are eminently capable of guiding a rocket.