Version: 12 SP
For context: I have a Page selector form control to allow the editor to select a custom page type from a specific path within the content tree. Some googling suggested that you cannot specify a start node for the Page Selector. As a compromise I was considering validating the field after a page is selected to check that the NodeAliasPath begins with the path where the custom page types reside.
This field has already been in use by the content editors, so I am reluctant to swap it out for a different form control.
It's not essential, but just for my own curiosity it would be nice to know if the validation rule can be done.
And to confirm, yes your summarised assumption is correct.