Ok I just read up on the El-Ninja (electric motorbike conversion that a lot of people seem to like, the first (famous) one of which was done on an old Ninja) and I thought about using the 'race option' (smaller, lighter, lower amp-hour battery pack that still delivers the 72v at a high amperage) as a basis for a plug-in/series hybrid bike. The 'race' option can apparently still do 12km or so in stop start traffic before it slows down.
So the plan was to put an 8hp (or so) engine that is purely used for charging the batteries in the space vacated by the larger batteries. I'm sure you couild get it to fit, as the original (and bulky) engine, gearbox, and radiator are all removed as part of the conversion, and the smaller batter pack would be small enough to leave room for an air-cooled motor and generator setup, especially if you mounted it partially outside the fram (to use the space free'd up by the radiator).
The motor/generator would be sized to charge the battery at 120% of either the average power used during a trip, or the power required to maintain 110km/h, whichever is greater. This would mean that the motor runs (at full throttle only) for about 80% of the time the bike is moving. An electric starter motor would be used, automatically controlled. Also, with such a large battery pack (check out the range hybrid cars have on EV only mode, it isn't that good, nor is the top speed), a plug-in mode could be used for shorter trips.
Rather than having a user selectable EV/Hybrid switch, the user would turn a dial to select what charge level is maintained. Effectively this would be a 'km for EV only' selection. Another bonus is if that trigger point is exceeded (say the trip was marginally longer than expected), the battery would only be charged enough to get home, and start using the (cheaper than gasoline) house power to charge the battery.
The main disadvantage to this setup is that it is less efficient than a parrallel hybrid, but it is significantly easier to set up and automate (those being the main contributors to cost and the amount of people that build one).
Thoughts? Or am I crazy?