Another way and using only TW tools is using "Text Areas". In this case two of them one over the other. The text area allows you to set an opacity, but as doing so will also fade out or reduce the opacity of the text, using only one is as far as I understood your question not what you are searching for. So create a text area setting the background opacity at lets say 50 for example or as low or high you need it to be set. Do not include a text in it, use it just as a background.
The text include it in the other text area you create. In this case the size has to be the same and it has to be placed exactly over the other, the first one. Under properties of the second one, choose no Background color nor Border color and set the opacity at 100.
Under "List Values" write the text you want to be shown. There you can have it linked to a scene, map, movie or to every scene, choosing "None" as source.
I hope this helps,