can one of the images in the lightbox be filled with text?

Not really, the lightbox is to open the image or the video in a larger size and provide some controls over them, but it cannot contain text only. 

Having said that, you can choose to have a separate image for that thumbnail, and add an empty png file, then use the description panel to add your text and select the option to have it shows up only in the lightbox, but note that the description panels will not be vertically centered like images and videos.