I'm not sure this really fits with what squishit does, since you need to alter the headers on the file response for it to work.
Typically this would be handled by configuring IIS handlers to do dynamic compression or with explicit code in your asset controller (if storing bundles in memory and serving through one).
Adding the ability to plug in a compressor may be helpful, but I am not sure how necessary it is given the other options available. I'm not sure its worth giving people another thing to potentially trip over when setting up their site.