With the Sony HDR-AS30V on my helmet set to 1280×720 at 120 frame/sec, the plates on passing cars remain barely readable (clicky for more dots):
Throttling the camera back to 60 f/s produces slightly better results:
The differences seem due more to changing lighting conditions than frame rate: the camera definitely produces better results in bright, direct sunlight.
These are about as good as it gets and, if you look carefully at the images, you can see plenty of compression artifacts that wipe out small details.
Equal-size dot-for-dot crops from the original 1280×720 images, matted together, and very lightly compressed because there’s not much detail to compress…
The Smell of Molten Projects in the Morning 
As you’ve discovered, normal HD cameras can’t quite do the job of capturing a license plate. The same is true with most surveillance cameras. They can’t quite make a license plate readable, even on a parked car.
I think you need one of these.
http://www.cctvcamerapros.com/License-Plate-Recognition-Camera-p/lpr-800.htm
That would increase the Extreme Geek factor of the bike!
It seems to work by illuminating retroreflective plates to boost them out of the background. That’s easier to do with a 75 mm (equivalent, I presume) lens, rather than the bike camera fisheyes, but a really bright amber LED array might do the trick.
Another few hundred milliamps shouldn’t be a problem… [sigh]
Perhaps you should reconfigure you quantization matrix for the video compression?
If all you want are detail on license plates of cars that are passing you, you can configure the matrix to heavily weigh the bottom-left of the frame.
Depending on the MPEG compression software, that might also encourage the rest of the frame to be 16×16 macroblocks, further increasing the number of bits dedicated to the area of interest.
With the right matrix, I remember that even 5 years ago I could pull readable license plates from a frame of D1 video (which is only 720×576).
Alas, the cameras are closed boxes with no access to their internal workings: they produce pretty images as long as you’re not too fussy about the details.
I’m sure there’s a Kickstarter project producing a Free Software action camera, but … [sigh]
High power infrared illumination might help increase the illumination/contrast of the retroreflective plates without otherwise affecting the image much.
I can feel science comin’ on strong: how much IR illumination does it take to overcome the IR blocking filter built into the cameras? Probably not too much for plates in the shade.
That’s assuming the usual hot mirror in the optical path; I am so not tearing those cameras apart just to mess with their optics! [grin]