I don't have anything that I've seen done, but surely it would be from a function here: https://restrictcontentpro.com/developer-docs/

Thanks Bobby.
Looks "non-trivial".
I'll explore replacing Restrict Content Pro with MemberPress, which appears to integrate with both FFPro and the Event Ticketing software in use on the site.
Thanks again,
Phil.

I looked at the restrict content pro developer docs. I couldn't find anything that defines their database schema, but I did find "there is a "memberships" database table and a "membershipmeta" database table." This means the plugin follows a standard WordPress metadata schema. You would have to examine the tables structures to know what fields are required by the plugin. You said you want to add a record to the tables after registration is approved. This implies an edit needs to take place at some point to approve the registration. If so, you could use the frm_after_update_entry hook to create the records in RCP.