The ability to enable custom actions on custom fields would be a fantastic addition to the custom fields feature.

For example:
there is a project_lead_email field on ProjectX, this feature would allow users to enable a "send email on update/resolve/etc" custom action for that custom field. Thusly, whoever is entered as the project_lead_email would receive an email when the issue is resolved. (for example)

Another custom field example would be that of "development_progress" (say, a 0-100 integer) that would change the priority of a bug, or some other action once the development process reached, 75, 80, etc

Extending the use of some of the bug_api.php (via email_api.php and a couple others) to custom fields would be very useful.