Yep that’s exactly what we want to do.
The catch is that it looks like custom user fields always appear as editable on the Discourse user signup form, even if you never intend for them to be accessible by users. We never want users to be editing their platform homepage url since that’s auto-generated by the system. However, since our users never actually see a Discourse signup form, this may be irrelevant.
To put this another way, is there a way to create custom fields that are only for internal use, i.e. that never appear on any kind of user-editable form in Discourse? I would imagine there are lots of potential uses for this in terms of integrating WP or other external platform data into Discourse displays.