Your setup is similar to mine. I don't turn my monitors off, I have 'em wired to a patch bay and I just pull the plugs and let 'em dangle!!! Same thing. Not very elegant, I agree, but easy enough.
Unfoutunately, your mixer (like mine) doesn't have the routing options necessary to do what you want to do. Here is a simple solution, though, if you don't want to go the patchbay route (which I love, BTW. Everything in my studio is permanently wired to it and I don't ever have to desk dive to dustbunny land). Get one of these:
It's an A/B audio switch. $14.95 at Rat Shack. Model number 15-1952. Put it in between the tape out jacks and the monitors. Leave the "B" side of it empty. Works like a switch.
Same as turning your monitors off- maybe a little easier on the power supply...... ;)