Here is a silly idea that seems to satisfy all of your requirements except for "efficient", and is one you could do for satirical laughs (at least from FE nerds) just to say you have a "Hybrid".
See, the difficulty with the normal solutions is that you need to somehow attach the electric motor to the transmission. This solution eliminates that hurdle. You don't need to fuss with that at all here.
The "solution" is to connect a fan... yes, a FAN, to the battery. Set yourself up with a switch on the shifter, and engage the fan (pointing backwards, obviously) when you need that extra boost to get going. If you are clever with electronics, you could probably automate it to engage at the appropriate times, just as in a "traditional" hybrid vehicle.
For those eager for a quick-and-dirty ("cardboard & duct tape") way to get started with this dubious hybrid technology, I suggest the easiest would be to simply hold a fan out the window during initial acceleration, or preferably, have a partner do it so that you can safely keep your eyes on the ScanGauge for critical measurements.
Of course any permanent modification begs the question of "Where will I mount it?" Since adding a bulky appendage to a vehicle's surface generally has a negative impact on fuel economy, this mod raises an interesting question of balance that might be answered quite hilariously by sophisticated computer modeling and/or tuft tests.
Regardless, perhaps this seed of an idea can further inspire more innovation in this direction. Pro Modders (as well as amateurs with money to burn) may wish to look into a battery-powered jet-engine hybrid, which has potential to be more efficient, and even more annoying to tailgaters. Not to mention low-power rockets, which might have the slimmest chance of being legalized if their use was restricted to assisting acceleration from 0-5mph. Any takers?
