I feel your pain. To avoid it I run an old design 3 channel amp (Mark IV) that has a great big global master volume knob in a very convenient location on the front panel -- something that's so big that it stands out.
That eliminates the problem of tweaking 3 different channel masters to get the levels balanced, which is decidedly inconvenient in a live setting. It's much easier to get my levels balanced beforehand, and just turn up the global master volume to accommodate any level changes that are necessary when performing. This saves me from the unpleasant experience of trying to tweak an amp that has too many interactive little knobs to allow dialing it in on the fly. (These later Mark amps can really end up sounding like ass if they're not precisely dialed-in.)
I don't understand why Mesa now gives us independent channel masters without a global master. Eliminating the global master seems to have happened when they started adding separate solo masters. Maybe they just ran out space when designing the front panel interface. The problem is that when you design an amp in a lab, it's an entirely different experience than using it on stage.
Although I guess one could say that you don't need a global master in a 2 channel amp, I've noticed that they've eliminated the big global front panel master control from the Mark V, JP-2C and VII. This makes no sense to me. On the V, there's global master but it's tiny. On the JP-2C and VII it's gone. It's as if they've gradually assigned less importance to a global master from one generation to the next, to the point that they just decided to eliminate it. I think that was a misteak -- I view the easy-access / big-knob front panel master as being so valuable that I won't consider "upgrading" to a newer Mark series amp. To me those front panels took a number of wrong turns as Mesa went from generation to generation.
All that is just a long way to say that I completely agree with your assessment that a global master is a must-have feature for an amp.
Would it be possible to mod your amp? Sure. Anyone competent at amp work can modify any circuit to add a global master volume. The real questions are who can do it for you, and where to put the knob. If the designers left you with a layout that gives you no place to put the knob then you're kinda screwed.
The problem with sticking a pot in the loop is that you can run into impedance matching problems. If you put a volume controlling device in the loop it really needs to maintain a high input impedance and a low output impedance so that it doesn't load down your loop. I'm not familiar with your amp's loop design, but I'm thinking that that sort of device really needs to be active/buffered rather than just a pot in a box.
