Interesting. Something you may want to look into then is the Briscoe Center at UT in Austin. They have in their collection the letters and papers of one William Preston Head, a surgeon (later head surgeon) of the 16th Texas Cavalry. There are several records among the sick and dead of that year that mention St. John's hospital. Others say they were "sick at Austin" or "Sick in hospital at Austin" and still others "in hospital at Little Rock."
Thanks for the information, it looks like I had my geography off a few points. I will have to check my notes to see who it was, but there was one officer of the 16th Texas Cavalry who was left behind at the breakup in charge of a convalescent camp. Most of this was probably due to sickness, but there was also a battle earlier in 1862 that is not widely known called "Cotton Plant." I know of approximately seven casualties. There were five regiments involved, including the 16th Texas. Some of the convalescents could have been the more seriously wounded from this engagement.