Switchable hubs have been around a while I think but they're not that common and I think they were considered unreliable (or less reliable) due to the extra bits and pieces needed to make them work. It's only ~1W off and 6W on anyway for their normal hubs.
Saw this:
https://www.tritoncycles.co.uk/components-c9/hubs-c120/sp-dynamo-hb-017-switchable-dynamo-disc-front-hub-p11393
@hippy @skinny would this possibly be of use to you guys for ultra stuff? Switchable dynamo hub, so that resistance when not in use is nearly that of a normal hub