Well, you have a real botanical problem to solve. I know a little about perrenials and don't know of any that will fit your specs perfectly! However, I would suggest you check out the "Dragons Blood" SEDUM family. These sedums are low, mounding, have a very attractive foilage, provide a nice burst of flowers in late June (which you would want to deadhead in July to keep the area neat!). There are quite a few really interesting varieties which you could mix and match to provide interest.
Sedums will grow just about anywhere. Just contact with soil will encourage rooting. They are not an "invasive" species (Ie., they don't readily reseed, send out branching rhyzomes, or flop over to spread.) However, once you plant them, they are difficult to remove because they break apart easily and as I indicated before they root easily!
You might want to plant dianthus (low growing variety) among the sedum. Dianthus is an "annual" that is a perrenial wannabe. If you are lucky it will come back next year and you will have contrasting flower color. It makes a really nice combination. I have a container of varigated leafed (?) sedum and magenta colored dianthus which has been beautiful all summer.
Lauri
