Well, it's just a free plugin (you can get it at http://sonictransfer.com/free-side-chai ... -mac.shtml), but I guess the answer can be about any sidechain compressor. I don't think the option with the aux bus will work because somehow Metro has to be able to access the input (sidechain) channel that the plugin creates when instantiated. So in Metro you will have to physically assign the output channel of one track to the sidechain input channel of the plugin.
Maybe this could be a feature request?
Here's a tutorial about the sidechain compressor: http://sonictransfer.com/side-chain-com ... rial.shtml
Sidechain compressing is cool...