I second (or third) the folding bike idea -- I keep mine under my desk at work, and don't even bother putting it in a bag. You'd be surprised how forgiving people can be of a folded bike. It's almost as if it ceases to be a bike!

Another option might be to ask about using the freight elevator. In my office building in a very fancy area of Shanghai, they don't like me taking the bike on the normal elevator, but have no problems with me using the freight elevator.