Thanks for the idea. The dual speed derailleur I used first worked fine at the expense of the chain rubbing against the inboard lip on the biggest two-three cogs in the back. Maybe I should try again with a serious angle grinder to remove more.

I then tried a triple, which did solve the noise issue… but the derailleur had an even harder time reaching the outter ring to the point I often had to get off the bike to move the chain by hand :-/

Besides the extra weight, a 100km test ride with the Alfine 11 left me more tired than riding the 2*7 derailleur solution, hence my looking at alternatives. But I appreciate the total silence, the single shifter, the ability of changing gears even at a standstill, and the lower risk of hitting the rear derailleur — it happened a couple of times, speeds would no longer work correctly, and I had to pull it outwards to fix the problem.

OTHO, the gear hub + cassette means two shifters, a derailleur prone to be hit (20" wheel), and a lot of duplicates/close speeds (~ 14 unique/useful combinations out of 3*9) :-/

"I mounted a 50/34 double with hollow spindle and external bearings, almost as much range and a far superior crank and bearing system in many ways" : What would replacing the square taper BB bring? Less weight, better performance? Would you have a link handy?