I think of a watermark or a scale as being two very different things. I trust you are aware of the option to display your logo in the title block as described at: bikecad.ca/custom_logo_in_title_block. If  you want to display an image like that outside of the title block, you could likely accomplish that objective by adding an image as described at: bikecad.ca/import_photo. If you simply want to show a note, that can be done as described at: bikecad.ca/adding_notes.

If none of this is quite what you're after, could you mockup a drawing, maybe with a pen or pencil and share it here. I'll try to add that option to the next update.