How about ajuga? It's good in your
zone and some varieties, such as the kind I have, grow very flat except when flowering. You just mow over it like grass. It's actually so prolific it can be a grass substitute. It's very hardy and is drought resistant. No drought here this year, however! Far from it...
http://www.thegardenhelper.com/ajuga.html Irish Moss is also good for your
zone and is really pretty. I've found, however, that it is slow to spread. Buying started
plants can be expensive for a large area. I've never tried it from
seed, but would love to use it as ground cover around all my shrubs. I'm not sure if its walk-on hardy, however.
http://www.swallowtailgardenseeds.com/perennials/moss_irish.html Good Luck