Thanks for the kind words (did I mention that Bob's Guides accepts donations?)
I'm afraid I can't help you much on this one. AFAIK, you can set a constraint based on the resource's fields (including the parent field), but not on a field of the parent.
There is an InheritParentTemplate plugin that would set the template to match the parent's template, if that would help.
You could create a new plugin based on the code of the InheritParentTemplate plugin and alter its code to do what you want. It should be fairly easy.
Maybe someone else has a better idea.
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