Just so I’m clear, what you want to do is:
- Have a WP custom field containing the “user’s platform homepage”
- Pass the custom field to discourse using the wpdc_sso_params filter as described here.
- Display the Discourse custom field on the user card and don’t allow the user to edit it in their Discourse profile (leaving “Editable after signup” unchecked)
If that’s right, that should work, assuming you don’t have any edit box for the WP custom field in Wordpress itself.
Just a note that staff can always edit user fields (i.e. custom fields), even if you don’t select “Editable after signup”. To see this in action you’ll need to test it with a non-staff account.