Brass does not really need any coolant - just cut it dry. Use a machinable brass like alloy 360 for best results.
Mild steel? Like what 12L14? Can you get your RPM down low enough? McMaster will have air misters for that application but they are messy. Still the RPM's may be a bigger issue on a router, then the rigidity of the machine will be an issue.
Imagine the Possibilities of a Creative mind