In my days as a check in agent someone once tried to check in a baby stroller. We had to charge him extra for it as he wasn't actually travelling with a baby. That wasn't UA though, but another star carrier, if that makes a difference.
I'd say anyone trying this is more than likely going to be challenged as these extra allowances are there for people who are travelling with a baby and need access to baby related things in flight. Yes it doesn't specifically say you need to be with a baby but it uses the words "in addition to the maximum" which implies you need to have a baby in order to get the extra allowance. Of course in this day and age no doubt they will only change the wording once it leads to a law suit, which probably isn't that far off.
