To be honest, I'm not entirely sure what the law is but from an ethical standpoint people with disabilities (even temporary) are entitled to reasonable accommodation from other human beings. What they are not entitled to is mind reading or telepathic gps-location services from an airline gate agent.
If the upgrade was THAT important to you I echo the sentiment of another poster in suggesting that you pay for the upgrade and not leave it to upgrade-roulette. Alternatively, if you simply wanted to make sure you were in the gate area early enough to hear any upgrade announcements, I would have thought you would have:
* gotten some crutches to help you move more quickly
* arrived earlier than normal, or
* asked for a wheelchair or cart.
-z