@Tetsuo_Kawakami , your hack works flawlessly, amazing! 
As such, I’ve reformatted the original post so it’s a more simplified setup process - I’ve introduced your method in step 4 instead.
I have also added steps 10 and 11 for those on Enterprise to leverage the new restriction on fields as well as obviously adding all this into a Bundle so that this system can be applied to multiple projects!
Hello all. I am following along with this thread and was wondering how I would do this when my subtasks are inherited from another project through Flosana. My rules aren’t working when adding the subtasks this way. My rules now are IF an item is added with certain parameters, then create subtasks (via Flosana) from another project. So when I set up the rule, I only get the 1 in the parent task, not the subs. I also added the subtasks custom field to both projects.