One outrageous idea would be to plant one Alexander McKenzie or Alexander Girault a few feet away from the fringe tree and train the tall climber into the tree as it grows. Then maybe when the white fringes form, there would be red flowers to go with them. Not many other reddish roses could pull this off. Alternatively, maybe two or three Red Cascade roses (ARE) planted eight or ten feet out from the tree might work. Red Cascade is vigorous enough that it can grow fairly near mature trees, though sometimes this stunts its growth a little.
I would be tempted to grow the cup-flowered tea rose Ducher (ARE) to echo the white of the fringes in another shape. The smaller polyantha Katherina Ziemet (ARE) might also look good nearer the periphery, with its small white button flowers on a dense little shrub.
I've grown very enamored of Calwell Pink (ARE) for the way it seems to settle in and grow like gangbusters in terrible, dry soil. It is also very nicely branched, and makes neat flowers through the season.
For a splash of color nearer the edges you might try Wine Cup (ARE), a dark red-maroon rose that is suggestive of the Texas wildflower it's named after. It is small, and to make much of an impact, I'd think maybe between 6 and 12 would be required. But I think its smoldering color - hard to get in roses - might be just the right thing to go with the pinks and whites.
Sounds like there are about 600 square feet here to plant. And if one gives up 300 square feet to roses, and each rose gets a generous 10 square feet, there's plenty of room for 30 roses. I think there might still be room for a paved path and maybe a small bench on the shady side of the tree, too.
Even though this is a rose forum, I'd be tempted to plant a few blue iris. The blue and white Clarence mixes really well with roses, is fragrant, and it re-blooms in fall. Perhaps some hollyhocks and larkspur could be added for vertical emphasis. The canna Pretoria might look really handsome as its foliage would echo the tree and the orange flowers would brighten things up.
If you want more splashes of brigher color in roses consider a Pat Austin rose, or Rise 'n' Shine.
I do recommend daily watering for the whole of the first season that roses are getting established, especially if the high temperature is above 90F. And mulch.
Good luck with the circle. I can't wait to see how it turns out.
Yeah, love the fountain idea! We have a large urn we picked up at Costco that we'll be placing in the center of the bed in front of the house and converting into a fountain. It's currently sitting on the front porch, maybe I'll go roll that big puppy out right now and get hubby fired up to get it done! 
