Here's what my friend suggests:

With only 15 days I would dump some of the stops so you could do better justice to the ones you do. Otherwise all they will be doing is traveling, and not seeing shit. I have driven enough of this to know that to be true. Right off the bat I would drop spending any time in Dallas and New Orleans. Too big, too messy unless you have some real time. I think Nashville would be next on the drop list. A great town, but not for what they want in a limited time frame. There is better music in Austin anyway. Keep in mind the great ones who live there aren't playing there. They are on the road. The rest are doing sessions.

From Austin you can take a nice drive to Hot Springs, from there go to Memphis, then go to Tupelo and take the Natchez Trace south to Jackson, then south, and if time permitted drive across Lake Pontchartrain to New Orleans, but not stop there, and swing back to Houston. Otherwise they could drive to Baton Rouge and swing west to get back to Houston. I include the Natchez Trace and Lake Pontchartrain for pure scenery. The Trace is a gorgeous drive in the fall. The Lake is big, and you drive across the middle of it. I wouldn't recommend doing it the way I did. At night in a storm. The road isn't all the far above the lake. This is far more doable, and leaves time for those happy little stops along the way.

His email is on the way in a PM.