This can be done in the setup delivery process of sending a coaching email.  Follow the steps below to filter by registration date:
  1. After approving your coaching email, click Begin Delivery
  2. Select the Current Event Participants radio button and click Next
  3. Select the participation types you'd like to be included in this email and click Next
  4. In step 3. Date Range, click the Checkbox "Send to current participants registered during the following dates" and input the date range desired
  5. Continue the delivery setup process to send out the coaching email