Hi,
Easy Profile extends Joomla user management, registration workflow is the same of Joomla. So all plugin that works with Joomla works also with Easy Profile.
To archive this you can create a Joomla user plugin or plugin/module to allow the access to registration page only if you have a OTP***, these solutions will works with or without Easy Profile.
For example exist a free plugin that allow registration only with a Passphrase (
http://extensions.joomla.org/extensions/extension/access-a-security/site-access/jrpassphrase), you can get this and customize to use OTP from this provider.