It has been proven that you can speak In Werewolf form (Sinding) and you can speak in Vampire Lord form (Harkon). So why can't we use shouts in those forms? We are not just normal Werewolves/Vampire Lords, we are DRAGONBORN Werewolves/Vampire Lords. Is the idea of a fire breathing Werewolf appealing to anybody else? I hope this ability is implemented in Dragonborn.
Due to your connection with the patron Princes of the forms as well as what I pulled right out of my bum, I assume you simply can't use shouts while in either form.