Is there a way in LRT to add a personal water mark at the bottom of a rendering or is this something that I would have to do within lightroom?

This kind of watermark shouldn't be added in Lightroom and neither in LRTimelapse, since that way it would be baked into the intermediary sequence or master video file. If you really feel you should add one, do that in the final video cut. That way you will still have the original clips without the watermark for other cases.