you mean that when we enter sub process the main process should be in state inactive-scope?
If so, I am afraid it won't work as expected, I really liked the idea though.
The reason of this is if you mark main execution as inactive-scope when sub process activity is executed then automatic signal after sub process ends will fail since checkActive is executed in end activity which is prior to signal of sub process activity.
Of course end activity code can be modified as well but I don't think it is a good idea.
In my opinion engine should prevent of signaling main process while sub process is active, otherwise we could end up in really weird situations where some of the executions are left and main process is finished.