Just a noob's opinion, but for my non-moto youtube work, I'm already used to recording to an external recorder of some sort, and syncing the audio and video tracks. In the studio I run a mic into an audio interface, and when out of the house, I can run up to two mics into my Zoom H1n pocket recorder. So, for motovlogging, it's just natural for me to do the same thing. Currently, I run a cable from my helmet lav mice into the Zoom in my jacket pocket. Having a dedicated recorder gets me adjustable gain levels, with audio controls I can actually access with my helmet on. And I also don't need a GoPro audio adapter, and don't have to mess around with cables running up to the camera.
After seeing MotoMaddox's pics above, I'm now thinking of doing something similar. Except, instead of mounting the recorder down by the exhaust, I'll just clip a second lav mic with dead cat filter, and run the cable back up into my recorder. The beauty of that is that both tracks will automatically be synced, and I can easily adjust the levels between them in edit