I've seen some that have big carabeener (sp) type closures that you wouldn't have to remove the bar for.
In fact, I think the makers of "Perfect Push-Up" have some (that they sell for use with their pull-up bar, but which don't have to be used with it). Check out Walmart, which is where I saw them.
There are also the "Ab-Originals" straps, but I'm not sure how they attach to the bar.
HTH!