Have you thought about making a Pololu-compatible board that would plug into the stepper shield? There was discussion about it on buildlog.net
Link to buildlog.net Pololu-Compatible board: http://www.buildlog.net/blog/2011/10/pololu-compatible-relay-driver/
The one that is depicted in buildlog.net is not a complete solution, it is just the part that plugs into the stepper shield. You also need another circuit where you have your mechanical or solid state relay to control the spindle.