Thanks Bob, your guides are always a great help. Unfortunately, in this case I can not follow your suggestion.
I can not "Create a Resource Group for each user" as there are about 300 users, and growing.
It would take a special ACL to restrict access to the resources that you own.
In this case I need only for Collections resources
I found this PL: https://github.com/modxcms/Collections/pull/248
If anyone has any other ideas, I think it might be useful for other people too.