IIRC Pebbled currently uses some ".summary" name/hint to get the notification, but that indeed just contains the "N new emails" text. But the other notifications contain both the sender and subject of the individual emails as shown on the events screen. But those notifications use a different name/hint, which you can easily get by running "dbus-monitor" when receiving the mails and reading the output.
For email, SMS and missed calls notifications I create both a pin on the timeline and a notification. This using the timeline spec. So I'm able to fully parse any JSON pin that the Android and iOS apps support and which can be used by other services to push pins to the Pebble. This is possible because Pebble actually publishes the description of the layout (all fields, which attribute id they have, and of which type they are) as part of the firmware update endpoint. And this also allows me to for example conditionally add the "createNotification" field to a pin based on whether the email already has been marked as read or not (when it has been read on another device). But as far as I've seen the implementation in RockWatch is hardcoded to those supported by it (notifications of a specific type or calendar events). So it doesn't allow the fully dynamic usage of the timeline pins.