As you have pointed out, the easiest way to access the airside Transit Hotel(s) is to have a boarding pass. I am assuming you are traveling on UA. I think you can do a city check-in the day before.

Another way is to make a reservation at one of the 2 Transit Hotels. Ask for a fax confirmation. Then, even if you don't have a boarding pass, you can apply to security for a pass (you need to leave your passport at the security office) and head airside. In the morning, you then come back out and exchange your pass for your passport - a bit of a tedius process, but it works.