That actually got me using my brain today!
But I figured out a workaround for this one. It's true that if you route the Rack's signals out onto individual tracks and then solo one of them the Drum Rack gets muted. You can set up return chains (those will be your Sub-Subgroups) in the Drum Rack itself and then route the individual signals to the specific return chain inside the rack. That means you create 3 return chains inside the rack, one for Kick, one for Snare and one for Cymbals.
You then turn up the send-A faders for all the Kick drums, the send-B for all the Snares, etc. What you then do is to create three ordinary return tracks in your live set. In the Drum Rack you then select A-Return as output for the Kick return chain you created in the Drum Rack. It also works the same way for the other groups you created. If you then solo one of your returns in the live set it works. It just means that you eventually use 8 Return-tracks as subgroups for drums.
The setup I built after your instructions looks like this (I just did it for Kick and Snare):
Hope that helps, somehow!