Shimano's patent for a new gearbox design.

[MichaelW2]

By "conventional attitudes" do you mean that, as the paper concludes, chain-line offset (normal length chain) has a negligible effect on efficiency? The enclosed very short chains will, I surmise, be at relatively low tensions, though (relative to the chain tension* experienced in a conventional chainring to sprocket combo). And since "efficiency varied with the reciprocal of the average chain tension [highest efficiencies occurring at high chain tensions]" this will mean losses in the dead in-line, very short chain runs, far outweighing the minimal loss of chain-line offset.
*About 200N at 200W power (big ring circumference 75cm, cadence 80rpm) and maybe 300N if in the small ring (same power). (Please say if my maths is way out.)

I don't understand how Shimano will deal with shifting and tensioning the chain inside the transmission box but I assume they have a solution and that it may have high enough tension for efficiency. We will see

 

[Tigerbiten]

Most hub gears have a limit for lowest gear that can damage the internal mechanism. Adding a Schlumph drive 2 ratio front end may take you beyond the limit.

You only need to worry about that if you fit the Mountain Drive as it's a 2.5x step down drive.
This ups the torque in the lower gears.
It's one reason that BB hubs are tricky to build as there's no step down from the chainring/sprocket before the gears.
The other two drives are step up so as long as the initial first gear is fine, then the whole drive will be fine.

Luck ............

 

[Globalti]

Where's @Yellow Saddle ? He's conspicuous by his absence.

Must be away riding his bike somewhere.

 

[raleighnut]

Must be away riding his bike somewhere

Or Drinking.