Hi Jason,
you're right in talking about the right way to simulate a feedback loop, namely using amesim rather than Pro/mechanism.
But in another postBoris Kirzhner told that you could use alo an external software in bundle with proE/Creo.
If this is not possible, or if you want to use an external simulation software together with Creo mechanism simulation, you should use Custom Load. Custom Loads functionality allows you to bundle external simulation application with the mechanism simulation. Your code will be called at every step of the mechanism simulation thus allowing simultaneous integration of mechanism's equations of motion with hydraulics equations.
So I'm a little bit confused.
Which are the real possibilities of ProE/Creo Mechanism? (Custom Load)
And it would be nice to see a real example of the use of this "Costom Load" function. On the forum and on the net I've not found anything, a part the PTC Help which, however, it's not a real example and, in my opinion, it's not veri clear.