Make slider always control channel strip across a concert

What you're asking isn't possible to do on a global scale; it has to be configured on a per-patch basis.

EDIT: Actually, you CAN do what you're asking IF:

1) you're using synths that have a multitimbral mode (e.g. Kontakt, Omnisphere, SampleTank etc), AND

2) these synths are configured at Concert level, AND

3) you're using channel strip aliases for each of these multitimbral synths for each patch.

Create the desired number of volume sliders for your concert using Layout Mode. At Concert level, create a single channel strip for each of your multitimbral synths. Configure each sound within the synth to respond to a different MIDI channel. Copy this channel strip to each of your patches BUT use the Edit menu option "Paste as alias" instead of the usual "Paste" command. Click on the channel strip alias for each patch and in the Instrument Channel Strip Inspector underneath the Edit Mode window, select the MIDI Input tab and then select "Multitimbral..." from the Input\Keyboard dropdown menu. For the Keyboard 1 input, select the Output MIDI channel that matches the desired sound's MIDI channel for that particular multitimbral synth. You will have to do this for each of your patches. At Concert level, map each of your previously created sliders to the volume level of each channel strip containing your multitimbral synths.

You will now have a set of onscreen sliders that control the channel strip volume levels for each of your multitimbral synths and because these sliders are mapped at Concert level, they will operate globally. If you need to make permanent volume adjustments per patch due to a particular sound being too loud/soft, make that volume change within the multitimbral synth for that particular sound rather than adjusting the channel strip volume slider.

Note that multitimbral synths have a limit to how many sounds you can load into each instance of that synth (usually 16, but some are less). Also, bear in mind that any channel strip at Concert level WILL sound for every patch UNLESS you set the MIDI Input for that channel strip alias to NONE (instead of Keyboard 1 or Multitimbral).

Also note that any changes to the MIDI Input section for each patch won't affect the Concert level channel strip, meaning that you can change the transpose, filter, scaling options etc on a per-patch basis for the channel strip alias of that multimbral synth. Finally, using channel strip aliases also means that you can use the same synth and sound for multiple patches without having to create a separate instance of that synth just for a particular patch...and you can still have your global volume sliders.

Another way you could approach this is by having submix aux CSs. Create a few Concert-level aux tracks for various types of sounds (one for pianos, one for lead synths, one for pads, one for effects, etc). Then route each of your instrument CSs out to their appropriate auxes. Then at concert level, you can set your slider controllers to adjust the output volume of those submix CSs. I've thought about doing this, but I think it's a little more complicated than I need. I do split off sound effects and backing tracks to different sub mixes, though, and have their own volume slider controllers.