I would recommend to keep the Master trim as it is.
Rather route the audio from each local rackspace not to the audio out but to the global rackspace where you go through a final gain block whose gain setting is controlled by a widget (also in the global rackspace).
In addition you could also use a gain block in the local rackspaces right before the “to global” block.
That’s way you not only have a “master volume” widget (with min/max option, or a special value curve), but you also have a “trim” control in each local rackspace to level out the particular rackspace volumes.
…building this will take less time than typing this post on my phone.
Further option: The System Actions plugin has a parameter for Master Trim. Add this plugin to the global rackspace and then link a widget to it. You can now apply the widget scaling.
That is not a workaround, that is actually the right way to do it.
The master trim is so called (trim) is intended as a means of quickly changing the overall output volume in the case where there is an issue with FoH (audio too hot, for example)