I do GIS work in Williamson county and have access to some data. If you are looking for specific areas I could overlay the drawings over parcel info and aerials, but I could not do that for the entire route.
I'd be happy to help you anyway I can. It may have to wait until MOnday so I can get some of the data files off of my work PC then compile what you are looking for at home that night. Or depending on what you need I may be able to access some data off of the internet.
Let me know.