Here is what I'm doing.
Select a different MIDI channel for each set of these. For this "singing" one it'll be channel 3.
Settings -> Midi ports / soft synths
Add JACK device, call it "singing ctrl" or something.
Route it's JACK In from your actual MIDI keyboard or controller hardware.
Add that new JACK device to Device port.
Click OK to close this window and save the new thing.
Add audio track, call it "singing".
Add MIDI track, call it "singing ctrl".
Set it to the selected MIDI channel 3.
Set it's Port to the above "singing ctrl" JACK device.
Routes it's input from the actual keyboard of the above "singing ctrl" JACK device.
Draw the track.
Monitor it, and only it, so that the TapeDelay2 plugin can learn from it later.
Add a synth Carla Rack-LV2, call it "singing echo".
Route it's input from the "singing" audio track.
Route it's output to your audio output track.
Route the "singing ctrl" MIDI tracks output to this synth on MIDI channel 3.
Open it's native GUI.
Add TapeDelay2 plugin.
Set it to the selected MIDI channel 3.
Learn your actual MIDI knobs for this plugin.
Leave the MIDI track monitoring on when you want to actually control it.
The one remaining problem is that there's no way to automate turning a plugin on and off.
Should work for other audio type ports. Though I haven't yet figured out what Audio Aux is for, there's only an output, there's no input for it to output.
Edit: I left out a line. I'll clean this up and repost it later.