Matt,
I tried this and noticed exactly what you mentioned. When I broke it down a little, it turned out that the jaw action was similar to packing, so I'd say that breathing this way amounts to one pack.
I've done some experiments using a Capri Sun drink pouch (very scientific) and determined that each swallow when I pack puts about 100ml into my lungs, so I think that you are probably adding at least 100 ml of air when you inhale through pursed lips (assuming that you are opening your jaw wide to do this etc.) However, the difference in fullness seemed to be more than the extra 100ml could explain, so there might be something else going on here.
At any rate, I'm going to try this next time out. I like to fill up for medium deep dives, but I don't like packing since it takes some of the relaxation out of the dive. This technique seems like a nice compromise.