Its not a license per se, but to call HS ball in Texas, you must be certified through the Texas Association of Sports Officials (TASO). They have a 50 question test that you must pass. I don't know how they would treat you in regards to the fact that you were not a member of any TASO chapter, but you can find out more by going to
http://www.taso.org, and contacting either Annette Stager or Tony Scazzero. They should be able to tell you what to do.
Where in Texas are you going to be? I may be able to help you find what assigning secretary to contact if I know who it is. If not, there are several other Texans on this board who may be able to help as well.