I used to use a technique when recording acoustic bass drum where I would set up a compressor with a tone generator on one side of the gate and the mic'ed drum on the trigger. Instead of clamping, the trigger would open the gate to allow the tone through.
The compressor in caustic seems to only clamp/duck. It's there a way to set up a reversed gate in caustic? I've had a little luck using the modular machine in but it's dirty.