I'm going to assume that by "presets" you mean Patches (as in MainStage patches)...also, I'm not really sure what you mean by changing "the presets of the mainstage itself (top keyboard)" if the top keyboard is supposed to be used as a "mere controller"? Sorry, but I read what you wrote several times and some of it is simply not clear to me.
Regardless, the part you say you have no idea how to deal with (sending "a Midi signal that changes the parameter in question that you want to modify") I already told you a way to do it, but unfortunately I'm not sure how you want to trigger that message, and this is what I was trying to get you to answer before.
Anyway, I've added a Scripter preset you can try out loading it in a Scripter MIDI Effect plug-in you load on a External Instrument Channel Strip in a Patch. It does the following:
When the On Patch Change > Send Expression box is checked in the MIDI Output tab of the External Instrument Channel Strip Inspector, if the Expression parameter of that Channel Strip is set to 127, when you change to that patch from another patch, it will send CC 113 with a value of 90.
It will send CC 113 with a value of 90 when you click the Send button in the Scripter preset.
It will send CC 113 with values from the Reverb parameter in the Scripter preset.
Maybe this will give you an idea of what is possible with Scripter.
J.
Send MIDI.pst.zip