Not disputing that this is a very much needed feature, but have you tried the
workaround that I posted in this thread?
1. Set up your audio track fed from your desk.
2. Set input monitoring to Auto, and leave Record and Playback faders linked as per default.
3. Insert the Digi-rack Compressor on the audio track. Activate the side chain, and set the side chain input to the same hardware input as the track it is inserted on. Adjust sensitivity so that it either attenuates or completely mutes the input signal to taste.
Thats it.
Make a time line selection with pre and post roll set up, as soon as you record the artist will be able to hear the playback during the pre-roll - which will be muted or attenuated as soon as they join in.
Playback works as normal - though if the artist plays along it would trigger the compressor.
You can have the compressor set so that it completely mutes the input, or if the performer wishes to play back accompanying themselves during a rehearsal, just attenuates live input & playback. And of course you can just bypass the plugin completely - so if you are recording multiple performers in one pass, you can bypass or activate all the input compressors at once - which is no more hassle than having a dedicated button to switch input monitoring on or off (though admittedly not as good as having a preference option for it).