I have one in the front yard. Its in a mostly sunny spot, decent soil (not too sandy, not too rich) with some hardy cactus, russian sage and salvias. Mine doubled in size last year (first year) and I know it bloomed for a little bit at one point but I honestly can't say I remember the color. I remember reading, too, that it could be white or it could be a blue/lilac color. Having said that, I don't think its really grown for the flowers - I chose it because its low-growing habits and color (you're absolutely right - this with a dark purple ajuga would be beautiful! Like I said, mine is with the dark purples of salvia and light blues of russian sage but a deep, almost black purple would be beautiful with this!).
I'm right on the border of zone 5 and 6 in Michigan. I'm sure I don't need to tell you what a crazy winter we had and, even though snow insulates, we still had a few weeks for -10ð and my creeping sunshine made it through. Never dragged a hose across mine, and its still quite small, but it seems to be very dense and matted so I would imagine, after a little maturing, it would be able to stand up against a hose.
If you already have them give them a try and if they don't work out then I have some false lamium I can send you... talk about a resilient ground cover (JUST KIDDING lol)

Just saw mine blooming the other day and they were white FYI. Just wanted to update you.