Get sheldon style 10lb wheel dynamo with DI2
That, or perhaps an 800gram ventless solar paneled helmet should be standard equipment.
I'm curious, what is the running voltage on DI2?
If its Lithium Ion recharge, am assuming 3.6-3.7.
It'd be so easy to double your milliampH with 18650 battery in stem.
These batteries, slightly larger than AA, will fit damn near anywhere. Seatpost, handlebars, stem, etc etc