Pebble has announced the release of a Watchapp SDK for OS X and Linux developers.
A watch face app: ● Is displayed full screen without the system status bar. ● Usually uses the minute or second handlers. ● Is expected to be very careful with its use of system resources (e.g. power, display updates) in order to preserve battery life. ● Can be made a “default” that is automatically displayed when the launcher menu times out.
You can get the update from developer.getpebble.com. You'll also need the new PebbleOS v1.10 firmware which adds support for third-party watchfaces created by the watchface SDK.