Dan .... relax - you're trying to reinvent an already efficient system. The sole purpose of the sparate outs is to expand your flexibility for a more elaborate mix. If you have simple needs, then the master works just fine. You seem to want more from the mix, so........ you NEED to accept and embrace the idea that you are in a "specialty" league, and consequently need to modify a few techiques.
The volume pedal is a passive, audio device with 2 SEPARATE in's and outs. Plug the bass into one, and the master into the other and route them to SEPARATE channels of a mixer. Now, when you need to make adjustments to all instruments at the same time .... it's as easy as moving your foot on the pedal.
Remember ... you can still make individual adjustments with the sliders on the keyboard, or at the mixer too.

Don't let this make you crazy. It's a good system, and it works the way it should. Remember, the key word here is "S*E*P*A*R*A*T*E"

Seperate means apart, and that's what you get when you use the special outs. Try the volume pedal first, and if it's too combersome, I have one more idea, but it's more drastic, and you'll loose all your internal effects.

Yamaha has a great idea on the 9000 series - you can take the entire signal from the kb, route it into a mixer (along with anything else in the mixer) and feed it BACK into the internal speakers, so the volume control becomes a master for ALL sources. Neat, huh?

Try the volume pedal .... it's an easy fix to a small problem. Another variation on the pedal is to use a very small mixer, that sits ON the kb. Then you have a master volume control at your fingertips again. Check into Alessis, Behringer, ROlls .... they all make small mixers.(food 4 thought)