The other things you could do is to build it with the dropouts off centre by 2.5mm eg 65mm CL to NDS droput and 70mm CL to DS dropout, then reduce the dish on the wheel by 2.5mm to suit. With the Alfine hubs flange spacing being 25.5 : 31.8 this will give you very close to even tension both sides (28.0 : 29.3).

Of course you won't be able to swap wheels without re-dishing, but is that ever seriously a problem? On my touring bike I built in a 3mm offset so that I could run Campy 10 speed with a 0.7 to 1 tension ratio. It takes about 5 minutes to re-dish a wheel.