You're right, of course, that it's not a very helpful name... but I can't think of a better one.
How about "tread dynamo" as opposed to a "sidewall dynamo". Anyhow, it doesn't matter at the end of the day what it's called.
Did you have to modify the rear fender to use this type of dynamo? I imagine most cyclists who would use a dynamo would also want fenders; I do. EDIT: I think JohnDThompson's photo answered my question.
I'm still waiting to hear about this
game changing dynamo they've eluded to over at the VO blog.