Off the top of my head, vertical is okay. I'll confirm that as soon as I can.
Okay, the Transportation Code doesn't stipulate a direction. It does, however, state that the details pertaining to the display of license plates are determined by "the board," and I have no idea where any amendments might be published, other than the Transportation Code. As nothing in the Code specifically states that it must be horizontal, and nothing specifically precludes vertical, I'd say that if you do get tagged for a vertical plate, and it's otherwise visible, illuminated, and up to spec, fight it. Make the court show you where it's prohibited. I doubt that will happen though, unless you've made someone look for an excuse to cite you.
