It might be possible depending on how you've got things routed but there is no way to adjust the ouput configuration of the built in metronome. You could always use a midi track with a cowbell hit or something and route the output of the VSTi anywhere you wish.
The only other way I can think of if you absolutely want to use the built in metronome is routing everyting else out higher number outputs and then sending your main output to the drummer's headphones. It's a pain in the ass anyway you look at it but the midi solution works like a charm (and you can do tempo event changes).
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