Sounds like, though, that you wouldn't want an outright memoryless nonlinear amp, which would cause mucho distortion of audio, but rather just a fast expander. So take a compressor and monkey with the feedback (or feedforward) so that above your threshold, your fast-as-possible detector is now used to increase the gain of your VCA or other controlled element rather than decrease it (like a compressor).
I would imagine there are some schematics floating around of d b x or even D*lby system blocks. In fact that meta thread with old manuals etc. I think includes some JBL/UREI relatively recent units that have expander modes alongside the compressor ones.