I have a bike with Alfine 11 and belt drive. I think I could use a 175mm crank, certainly a 172.5, rather than the 170 Alfine comes in (height 6'1" frame size 60cm) and going a little lighter and prettier with Ultegra or Dura Ace appeals to me.
All are two-piece chainsets compatible with the Hollowtech II bottom bracket. All have 130 BCD. Seems like a straight swap is possible. The only thing I can thing of that might go wrong is belt alignment - is the offset of the larger chainring mounts on the road sets going to be the same as the mounts on the Alfine?
I was able to find that the quoted chainline for the Alfine chainset is 42.7mm - I can't find that figure for the other options.
new information: Gates produce a bunch of shims and spacers - mentioned in the engineering manual but I haven't been able to find them for sale anywhere. It looks like with this kit an Ultegra chainset will work but it's an expensive experiment without knowing for sure. I'll have to email Gates to try and find the kit, then buy the chain set if the rings look like they will correct the chainline - which from all information must be as perfect as possible.