I have considered that during my quest for efficiency but I decided it would be contrary to the goal (non-duplication, resource usage efficiency, management simplicity). It also requires the AppVM to have networking which is a problem for offline and firewall-restricted qubes.
So far, the best (or should I say “least bad”) approach (in regards to efficiency) which I have found is the decremental approach. I have optimized it even a little more than what is shown in the post I link to, but I have not published anything yet, because it is work in progress. So, meanwhile, I keep looking for a better way because I still have this security concern.