Its possible to control your FX speed with tapping bpm in onyx and with the help of an external application (e.g. show cockpit).
This is possible because Onyx can output tap bpm via Ableton link since 4.10
Go to your Library-> Input sources and create a new one.
Under Beat Provider use Source Type Synchronization and then Ableton link. Make sure you turn on the output of Ableton link.
Create a override cue on your FX Speed and put that on a playback fader. e.g. 20
Show cockpit has to convert the bpm from Ableton link that onyx sends to a fader value with a Lua script and send that via OSC to your fx speed fader.
Here is a tutorial from show cockpit:
You can now Tap the fx speed! 