I have been looking at the source some more and I think I have found a solution to my own question.
What I need to do (I think) is to create a plugin which defines a new 'account' action
<action name="account" class="com.jivesoftware.community.action.CreateProfile">
<result name="cancel" type="redirect">index.jspa</result>
<result name="success" type="redirect">account-success.jspa</result>
and uses an extended version of the CreateProfile class to call my modified sendValidationRequest(User user) method.
If anyone things I'm on the wrong track let me know.
We have a similar request to provide a mechanism for an admin type user to validate registration requests before sending a welcome email to the new registrant. How did you finally go about this?