If there were a way to insert an address that can't load you'd be set. I use this tablet while on break at work and their network blocks Gravatar for some reason, so when I'm at work this particular site never finishes loading and thus the toolbar stays visible. (Actually Opera does eventually give up, but I have to stay on the same page for a couple of minutes for that to happen.)
No, I haven't found a setting. On a 10"+ tablet with 1920x1200 resolution there's no reason I would need to hide it except when viewing a 1080p video, but I'm stuck with the current behavior too.