It seems to be now available and functioning. The coupons also got "moved" from their usual place in the account to the very botton, where they now have their own section (it caught me off-guard at first, because there's still an indication that there are upgrade coupons at their usual spot, but then there's no coupon number showing up there and when you go in that sub-menu they are gone :P).
What I can't test is how it works with itineraries that include multiple flights, if one only requests an upgrade for one segment. Will the system be able to split the booking like phone agents did so far or will it demand as many upgrade coupons as the number of flights in the itinerary (and if the latter is the case, would a phone agent still be able to help)?

For flights already on wait-list it says that the booking class is not eligible for an upgrade either (makes sense, since they do have a "double-segment" showing up in I).