A feature has been added to the "Compose New PM" screen, allowing you to include all storytellers when sending.
Right below the "To" and "Bcc" fields, click on the "Show Membergroups" link
This will drop-down a "Send to Storytellers" option as shown in the image below, with check-boxes to select use of either "To" or "Bcc" at your discretion. Note that prior to sending the PM, you will not see the storyteller account names populate in either the "To" or "Bcc" fields.