Uhmmm... not sure about that. Never say never, but LR, by default doesn't include a feature like this. Maybe with the aids of some plugins to be downloaded somewhere.
A partial solution can be to create different folders named, for example, as the "watermark-to-be-used", than you can move the pics inside the correct folder. In this way you'll have a specific order and all the same images to be signed with the specifc one.
Though, only down side is... you'll have to run the export process for each one.
grazie Alessandro! Yep, it's a partial solution but still better one then doing it manually for each and every pic... you made my think: what about using a tag so I can filter them in the gallery, select and export?That would allow to keep the pictures together in the same folder.
Well, yes, that's another work-around solution: you can use whatever TAG, text, stars, flag and so on you can think of, then using the filter and export as you wish.
Only "down side" is ... you'll have to remember whatever you use for the specific image, but it can be avoided if you use clear and always-the-same text