If the high hat is in a separate layer, you can adjust the layer volume in the Score Manager. If you click on the triangle next to that instrument in the Name column, you will have access to the layers.

I often encounter the same problem, when "Drum Set" is selected. All the sounds of the Drum set are concentrated on one midi channel (10), so one cannot change the balance between the various components of the Drum set. Impossible to split the sounds to different layers. They share the same midi channel (for percussion).

Your idea of another staff (hidden ?) with just the element you want to put in balance seems to be the only solution I can think of. That staff could use another percussion midi channel (26,...). But I could not get it to work.

I'm curious to see if someone comes up with a solution.

Admittedly, I am a playback Luddite. I only use MIDI, and don't even have Garritan loaded. If 10 is reserved for percussion, in the first bank, could you change the bank and use it again?

One can load multiple instances of Garitan Aria (or SoftSynth) and assign different layers to different instances. So as long as one used the louder instance only for the drum sounds one wanted to sound louder, the biggest problem one would likely have would be stemming.

Admittedly, it's a lot easier in VST.

I still don't get how you could "isolate" a hi-hat for instance in one layer, because all the (many) drum sounds are part of the single Drum Set instrument (each midi note produces a different sound).
P.S. : I also use Midi playback, but via an external synth.

(Notice that the Softsynth for layer two is a different Softsynth: Softsynth 2.)

Of course, if you are using an external synth, unless there are some acrobatics you can perform there to isolate voices, you would need a duplicate synth for the Pedal H.H. (or any other sound you wanted to isolate) to send different velocity/volume to just those notes triggered by the second layer.

This is not something I do in my own writing, even with VST playback. It's far easier to use hidden accents, dynamics, or tracks to bring out specific notes.

The idea to use different dynamics (hidden) for specific layers is worth exploring, as long as a specific instrument (hi-hat) is placed in a specific layer, all by himself.

The Expression Assignment window allows indeed an expression to be applied to the playback of a specific layer.