Is it legal to use a camera on your helmet or bike? So far I haven't found any law against using a helmet-mounted camera except possibly in Australia, so this is derived from articles on dash-cams.

The legal issues where they exist are all derived from privacy concerns and despite having common GDPR standards, vary enormously between countries.
Starting with where it's legal:
UK, Spain.

OK but restricted:
France and Belgium, OK to record but no public use. i.e. no youTube.
Germany; public upload only after all personal information is removed e.g faces and number plates. So fine for recording those scenic rides on empty roads.
In Italy, Norway, Switzerland, Sweden, the Netherlands, Denmark, Malta, Bosnia and Herzegovina, and Serbia it's OK to record but varies after that; I haven't found any specific detail for these.


Banned and don't even think about it:
Austria and Portugal where it is illegal to even own such a device.


Motorcycle Rider (Summer 2021)
Viatech Dash-cams around the world
Schoenherr - dashcams an austrian perspective 
Thinkware dash cam - dash-cams in Europe  
The Chauffeur - is your dashcam legal

NRMA: Is it illegal to attach a camera to a motorbike helmet?