would like to know as well, right now im using the jp yellow springs.

Rock River told me their trigger would not work with 9MM hammer.

JP said their single stage will work with Rock River 9MM hammer

The RRA 2 stage would work if you had your bolt ramped.

 I have an Olympic 9mm upper on a PWA lower with a Rock River two stage installed and it works fine.
You didn't state what type of upper you have.

I've recently purchased a RRA 9mm upper (Colt style) so I can take my AR to the local indoor range.

I haven't had an opportunity to test fire it (still waiting on other parts), but I've cycled the bolt (non-ramped) with my Knight's 2-stage trigger.  The hammer locked back fine, without binding.  I've dry-fired many times, and it seems solid.

I'm slowly building up my 9mm, so I don't expect to shoot it anytime soon.  Because of this, I can't give you a definite answer as to whether or not it will work.  However, the problem with 9mm/5.56 seems to revolve around the 5.56 notched hammers binding/locking the action in Colt-style systems.

Short story, if you have a Colt derived 9mm, go with a standard 9mm hammer, or get the bolt ramped to use any 2-stage trigger system.

From what I've heard, the Oly system allows any type of trigger to be utilized.

In my case (non-ramped RRA 9mm), the Knight's trigger seems to work fine.