Chainring offset - chainline - of Alfine vs. Ultegra/Dura Ace
The Alfine has a chinaline of 42.7mm for a single crank and the Ultegra 43.5mm for the double and 45mm for the triple.

For the double the chainline is measured halfway between the 2 rings so the outer ring will be even further out, but the inner further in.

Sheldon says "With typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm".

1.7mm or 3.3mm both sound like a long way for a belt drive to me. But I'm not an expert there. You my be able to use the BB spacers to offset the whole crankset 1.7mm towards the drive side, but I'd rather not go down that route.

This is all assuming there's no extra molding on the cranks or rings to get in the way.