Hi Paul
What any individual app will be able to do after watchOS2 will depend on the developer (you could contact them to ask).
However, the following confirms that, subject to their developers employing them, third-party apps will be able to access the heart rate sensor and other hardware features:
"Apps can now take advantage of features like the Taptic Engine, Digital Crown, accelerometer, heart rate sensor, speaker, and microphone. For example .. Strava uses the heart rate sensor to gather and display data during workouts."
From: http://www.apple.com/watchos-2-preview/