To begin with, the app store policy forbids linking or 'channeling' users to an outside location from inside an app. So MS cannot put a link in the app, or even tell users where to go.
That's why I didn't say to put the links and words in the App, just on the SkyDrive homepage where Apple has no right to control what to be put there. Maybe SkyDrive team can put the words on their blog too. I'm sure Apple has no ground to control what's on the other's blog. (Btw, what will happen if the App includes little blogroll to their teams blog, and the blogroll "just happen" to have article explaining this? Does it violate the rule?)
2/. You cannot offer a cheaper subscription than the one offered inside the application. This means that MS cannot hike the price of the in-app subscription to claw back the 30%.
That's the part difficult to circumvent... I'd like to see the actual words of condition. If the words doesn't contain clauses that "prevent authors to offer subscription outside the App ONLY", maybe Microsoft could just pull away the button to renew subscription inside the App.
Btw, what would happen if Microsoft offers SkyDrive discount to people like MSDN subscribers? The current Apple account has no indication for such status, therefore offering lower price on the web based on this ground seems reasonable to me.