Sorry, my fault. I didn't read the article http://developer.android.com/training/system-ui/status.html carefully enough ...
The new method of hiding the status bar is only for Android 4.1 and higher :-(
I use the Garmin Tempe, it is slightly larger than a CR2032 battery and weighs only 10g. It is installed on the outside of the helmet, so it is not affected by body temperature.
Status bar: I think we can provide a setting for that (Configuration >> General Settings)
ANT+ environment: What sensor would you use for that? Would you really carry an extra sensor to get the temperature? If you have a smartphone with build-in ambient temperature sensor, you could use use a handle bar bike case (to avoid influences from your body temperature) to get pretty good temperature measures.