You can probably do this with Xenakios' extension. A search should dig it up pretty easy. It has actions for toggling the master sends. We definitely should have a native way of doing this though, IMO.

You could do it with a macro probably too or use one of the js plugins and stick the control for it on the front panel (although you would have to turn the master into a quad track).

Download the latest extension from here:

Yeah, what Bevoss said. :) Just make a new custom action that has Toggle Master track send 1 Toggle Master track send 2 ..and set up the master in/out with 2 outs(monitors and cans) with one muted. The macro will then toggle them. ;)