It is expensive, but it's available:
Redrock Micro - Live Lens MFT

It avoids flattening the camera battery powering the lens from its own battery.
But it won't power focus by wire only lenses, only those with manual focus.

Kipon say they will provide one in the future:
KIPON released electronic adapters,using Canon EF lens on NEX&m4/3 mirrorless cameras-KIPON adapter

But their current offering has a diaphragm built in and suffer from vignetting.
So this one will be much better, using the in lens diaphragm.

And it's using the native mft controls from the camera for aperture.
So it's taking power from the camera battery.
I wonder what will happen if I put the EF 300 f/2.8 on that adapter.
How long will the camera battery last?