No, but if the government allows other forms of expression in the same medium, then they can't engage in content based restrictions without meeting a constitutional test of "strict scrutiny". As it currently stands, its perfectly legal to own and display a Confederate Flag. So if Texas offers groups the opportunity to make custom license plates, it can't discriminate solely based off the content without narrowly tailoring the exclusion.
I believe another poster in the thread mentioned they had prevailed in other states, I am guessing the basis of their argument was what I just laid out.
We may just have to disagree on this, but I don't see the government's failure to offer on option the same as content based restrictions. An individual has every right to plaster their vehicle with confederate flag bumper stickers, so long as it does not interfere with the safe operation of said vehicle.
I can't see how you can force the government to adopt an medium for your expression... especially if certain segments of the populace would view this as fighting words. I don't think I'm alone since some Circuit Courts have upheld suspensions of kids that have worn clothing with confederate flags to school. Of course, the Supreme Court remains silent on this issue.