Custom field validation hooks
There do not appear to be any hooks that can be fired between the custom fields entry page (confproduct) and the shopping cart. While a regular expression is good to determine if a field meets criteria, when dealing with provisioning it may be necessary to validate the user's selection externally.
Using the hook documentation as our example, and referring to the hook_create_forum_account sample code, we have the following issues to contend with:
1) How do we validate that the user's desired forum name is available?
2) If it is not available, how do we alert the user?
3) If it is not available, how do we return the user to the confproduct page to update their selection (preferably with the alert message from #2).