It was suggested I make this a feature request, based on this thread:
I have multiple font choices, so unfortunately the suggestion in that thread won't work.
In the product builder you can set the text to uppercase and then hide the text-transform tool via the UI composer.
While I'm currently doing that for existing text elements that the user can modify, no such option exists for the Custom Text option. Example:
The 'Add Text' module is the standard Custom Text option. There isn't a way to force that to be uppercase. I did try the CSS way by using text-transform: uppercase in the text box, but it still shows up in lowercase when they add it to the design.