You have a few options.
You could create a subgroup before your master fader for all audio tracks, and route all effects returns directly to the master fader. Then mute the subgroup to hear only effects returns.
OR, as needed for soloing, assign the out of the effect return to an unused (perhaps digital?) output. Then open your i/o settings and swap the digital output with your stereo out - Thus effectively not using up any more analog i/o.
And of course, there's pre-fader sends. Not useful for a full mix, but since you're on an HD rig, you could change a send to pre-fader, then use the "copy to send" function (⌘+option+h) to correct the send level.
But the easiest way you've already said; just use another pair of outputs.
By the way, I'm not in front of an HD rig at the moment. Is it possible to assign the PFL path to the main output?