Soundspectral (Rene Ebenhan) has released OutMax VST, a free plug-in that gives you control over (mono) channel output.
With OutMax you can monitor stereo channels and hear just left or right in mono, but on both channels. It also features a limiter and maximiser, with clipping protection for output over 0db.
CPU consumption of the plug-in should be low (developer measures around 0.5% on a P4 2GHz).
The OutMax VST plug-in is a free download.