So I emailed acmilan.com and asked them about foreigners buying tickets in advance. I was a little surprised that I got a response in just over an hour.
There's a new service that handles this:
http://www.mpestero.com/index.htmLooks like it's possible (once the season starts) to get tickets well in advance, etc. The only hangup is that you need to do bank transfers and can't, for example, pay by credit card.
The site is in Italian and English.