Instead of using the base parent frame of the addon to create your FontStrings, use the frame that will be the parent (frame you want the text to overlay) of the FontString itself.
from:
Code:
UI_DisplayFrame.textFrame.title = UI_DisplayFrame:CreateFontString("buttonTitle", "OVERLAY");
to:
Code:
UI_DisplayFrame.textFrame.title = UI_DisplayFrame.textFrame:CreateFontString("buttonTitle", "OVERLAY")