Here's a JS plug in I wrote that can do this. However, there seems to be some latency going on that I haven't found a way to get around (yet).
You will need a MIDI loopback device for this to work (I use the free MIDI Yoke). To install the plug in, download it, remove the .txt-extension and then put it in the ...\Reaper\Effects\Jonas -folder.
1. Put the attached plug in (Audio to CC utlity) on the track that should control the eq and lower the theshold to maybe -20 dB. It might also be a good idea to float the Audio to CC utility-fx window. Route this track to "MIDI Hardware output: MIDI Yoke NT: 1" and "send on all channels".
2. On the second track load ReaEQ (or whatever you EQ choice is). Hit play (important!) and click on the "Learn" -button in the ReaEQ window (it's in the the top right corner if you have the fx in the docker) and choose which parameter to control (for example Band 3 (band) - gain if using ReaEQ).
3. In the MIDI learn Window the CC number chosen in the Audio to CC-utility (16 is the default) will appear in the "Command:"-line - just click OK.
4. Finally, adjust the two sliders "CC value when audio is below threshold" and above "CC value when audio is below threshold" in the Audio to CC utility so that the EQ band does what you want it to. (If using ReaEQ set for example the ...below...-slider to 32 and the ...above...-slider to 20, this should give you a 4 db cut everytime the volume on the first track goes above the threshold.)
Let me know how it works! (And I'll do my best to tackle the latency issue...)
And yeah, your MIDI loopback device needs to set to Enabled+Control in the Preferences\Audio\MIDI devices - see the attached picture.
Regards,
- Jonas
PS: And do try the ReaFIR alternative, I know I will
: DS
PS2: if you arm the proper envelope this will write to the envelope as well : DS