If you have no other source of purple light that might cause issues, you could make your text translucent and then read the SceneColor node. Then you could use the “Chroma Key Alpha” node to extract a mask based on the purple color and use that to reveal the text. The materials where this text appears may have to be fairly desaturated for this to work.
You could also just do it using a spheremask from the light position by setting that using a material instance dynamic parameter, but that method will not support shadow casting which is why I would maybe try the first one.