So to understand this, it is specifically the result of an export that you need to review - showing visible watermark - so syncing to cloud would not serve?
One option comes to mind. I only dabbled with this slightly, some years ago - but a Flickr.com gallery includes for a viewer of the images (within Flickr) to add comments. I don't mean saving an intermediate JPG and uploading that, I mean exporting directly to Flickr via an export plugin. For speed, only a very low resolution version will be needed and the Flickr settings can keep these images private to you.
There may be other options with the same functionality, but I don't have experience of those.
With Flickr, under this specific export plugin, a connection is made whereby web viewer comments synchronise themselves back down into the Catalog's Metadata panel.
If your overall set of images is defined as a Collection (say), you can make a Smart Collection whose rules are: membership in this named Collection AND a certain word present in the relevant metadata field.
In practice: perhaps some images turn out better if their watermark is positioned on the left: these images can be commented as "left" within Flickr. Then the above Smart Collection's selective content can be bulk exported normally, with the watermark set left. Then this Smart Collection's rules can be changed (in this Collection AND not showing "left"), then the remainder of the overall Collection can be bulk exported with the watermark set right.
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