Your scenario can be easily solved with the GRX-IRI
Run a MUX link cable from each GRX to the rack, or wherever you have installed your Pronto's base station.
You will be placing an emitter right on the back of this unit INSIDE the box, but since this is an entirely low voltage box, there are no issues in doing this.
The GRX-IRI is one of only two Accessory Controls that allows use of the new Pro-IR commands that allows you to directly control scenes, master raise/lower, and all off to any single GRX on the MUX system, as well as to all GRX units on the MUX system.
The other Accessory control is the SeeTouch SG-PRO
, which is also an attractive 4 scene plus all off control with master raise/lower. In comparison with the GRX-IRI that can be accept a stick on emitter, this unit receives line of sight only through it's IR receiver on the face.
These two controls are the ONLY devices that allow this as of today, and this is the best way to get around the situation Alan describes above without relying on an advanced routing scheme from the base station of a powerful remote system like the RTI RP-6 or URC MRF-300 bases. (I am personally not as familiar with Pronto base programming, perhaps someone can elaborate)
The biggest advantage is that if you ever change remotes (and we never do that, do we??) then you will only have to address the GRX-IRI instead of re-creating the entire elaborate prgram in the new system.