It depends how deep in you want to wade.
In addition to the artists you've mentioned, Johnny Winter's early recordings (collected on the compilation "Birds Can't Row Boats") are worth including - in fact I'd say they're as good as any of the acts you've mentioned bar the Elevators and the Golden Dawn.
Then there's the Conqueroo, an Austin band featuring Elevators co-writer Powell St.John, whose live act was, apparently, legendary enough to make them local heroes at the time - but who sadly only got it together to release one single during their lifetime the marvellous "1 to 3"/"I've Got Time".
Endle St.Cloud's one album for International Artists ("Thank You All Very Much" - the last LP IA issued) is an interesting slice of hard bluesy psych, although not on a par with that label's best releases.
Not quite sure whether or not the Sir Douglas Qunitet qualify as psychedelic, since their sound was more of a rhythm'n'blues/Tex-Mex hybrid, but they were from Houston, existed during the same time frame and certainly shared more than a few bills with the major psych acts in the region - so it's your decision whether to reference them or not, I suppose.
If you haven't already, it's worth tracking down a copy of "Acid Visions -The Complete Collection : Best of Texas Punk & Psychedelia", a 3-disc set contains that contains tracks by most of the the artists I've mentioned plus a whole slew about whom I know nothing besides the songs included by them. there are two more three-disc sets in the same series, but Vol.1 is definitely the one to go for if you want a primer - the subsequent volumes contain way too much plodding blooze-rock filler for my liking.
As for more modern stuff like the Buttholes and the Black Angels ... you probably know as much as me in that department, although I'd definitely second both of those choices (and possibly Jandek and Daniel Johnston too?), and would be interested to read about more.
Keep us posted as to how the article goes.
