1) Their culture is fascinated by novelty and innovation. Any peculiarities of the alien form or culture would be therefore more appreciated and embraced. In fact, you'd probably see a new fad in Tokyo of dressing more like the Betelgeusians by the end of the day.
2) The Japanese tendency towards proper etiquette is well-known. (We wouldn't want any British yobs chucking lager cans at the landing craft, for example). We could count on them to at least make a decent showing of human propriety.
3) They have a wonderful tolerance to the grotesque. This is the culture that has given us the terms manga, guro, yaoi, seppuku, and hentai tentacle rape. The aliens could be bloated, screeching sacks of bilious ooze-- I doubt one of the delegates would even raise an eyebrow.
4) You know as soon as they cast eyes on any sort of interstellar craft, within days they'd have a plan to make one smaller, cheaper, and more fuel-effective, thus ensuring humanity's place in the competitive spaceship-market.
5) The collective cultural memory of Japan has already experienced the shock of meeting "aliens", thanks to Admiral Perry landing on the island in 1853. The disorienting steps of meeting highly technologically-advanced "outsiders" are therefore not going to pack quite the same wallop.
What do you think?