I wouldn't have the five year old hanging around at the hospital, he'll get bored and you'll be worrying about him instead of focusing on you. Let him have his fun day out and then come back to meet baby brother. Of course, unless this is a planned Csection, you can't really know exactly how things will go, so you just have to be flexible.
On the room sharing, I'm guessing big brother will probably sleep through baby noises. I always thought our second would wake our first with every cry, but kids don't have those sensitive "mommy ears" that respond to every noise the baby makes. Of course, they didn't share a room, but were right next to each other with doors open. But there have been times more recently that they have shared a bed, and the toddler waking and calling (loudly) for me didn't wake my oldest.
Good luck, and enjoy this time. It has been my greatest joy to watch my kids develop a relationship and love each other!