OK look at this little
"baby gen" - according to text in article it delivers 6V@.330mA - now go figure and put one on each wheel and VOILA - you have 6V@ over 600mA - now that's enough power to charge a couple 26650 Li Ion cells in a couple of hours - and a single 26650 can supply a 200 lumen light head several hours.
This whole thing is completely doable - I'm guessing there just isn't enough "geek" utility bike market to warrant a "start up" venture. For one thing, first you have compete with the "general" bike-light market, and then select the slim piece of that market that cares whether or not they have to change batteries or charge batteries often. I'm guessing that' s just not enough market share to warrant some one putting the piece together, branding and advertising it.
But a starting place would be to take what you can get from one of these existing hub setups and expand it's possibilities. For instance, have it perform double duty by charging a battery for a tail light during the day, down hill, and then have all of its out put go to the head light when needed - after dark.....