I have a presonus firepod, logically it can have a seperate stereo mix for each of its 4 stereo outputs (or alternatively, 8 individual mono mixes based on how sends are set up)
Sorry I'm not by Sonar right now, but I would think that if Sonar gives you the option to send different track/bus outputs on your interface (for example, on my Firepod, headphones and main output are combined, but if I use the second stereo pair of outputs for my monitors, I could route the metronome to just the "main" output along with the mix, and then run the mix through the second pair of outputs to the monitors without sending the metronome there) so you could do some trickery there to make it work.
My method might be extremely wrong but it's how I WOULD do it if I wasn't so savvy as to just hit the hot key for the metronome when I wanted to turn it on and off ;)