I use a small busbar kit in jboxes all the time. I like it when you have 6+ EGC coming in, it's better than putting a bunch of blue wirenuts together. As far as listing, I read the directions in one of the square D kits, and it said that if the enclosure has no holes to attach said bar, drill them out and attach the bar. It does not list what enclosures this can be used in. I think that you could argue this as being listed to use in a gutter or box.
Besides, it really is a nicer, more reliable way to do it.