I think the issue depends on "what do you mean by master fader?"
I saw a Leprechaun console that the sound company used with Bill Cosby, where they did in fact have a master channel that was able to have dimmer control over everything.
In MyDMX, you can't really have a master control that behaves as a GLOBAL, but I have some ideas that might help. Examples work best.
I'm going to limit this to LED fixtures I own just to narrow things a bit.
I have 8 ADJ 64 LED Pros. Each has their own address. Using a nanoKONTROL and MyDMX, in Bank 1 on the nanoKONTRO, I assigned Fader 1 to all red, 2 to all gree, 3 to all blue and 4 to dimmer on each light's assigned DMX channels. I get global control over all of those. To do this I had to repeat the process on each light: Go to fixture channel, right-click, assign MIDI, move fader- learned. Repeat per fixture.
So, if you want that kind of control, then sure.
Keep in mind:
You can assign the same MIDI control to multiple channels. You cannot assign multiple MIDI controllers to any channel