Binding preview of an image

The only way I know of that a plugin could manipulate watermarks is by creating a watermark preset. These presets are stored in the Watermarks subfolder of the LR settings folder, e.g.

/Users/john/Library/Application Support/Adobe/Lightroom/Watermarks/My watermark.lrtemplate

The .lrtemplate files are Lua-syntax files whose contents should be easy to figure out. I haven't tested it, but I'm assuming that watermark presets are like any other kind of LR preset -- they are read once when LR starts, so if a plugin adds or changes a preset, the user will have to restart LR to force it to read the new .lrtemplate file.

A plugin can preview a watermark preset using LrExportSession to export a sample photo with export settings that include the watermark preset.

The plugin can display a photo that's not in the catalog using the LrView "picture" control. It's not well-documented, but a plugin can set the control's "value" property to be a string that's the full path of the photo to be displayed. It can also set it to an LrView.bind binding, letting the plugin change the picture on the fly, e.g. 

f:picture {value = LrView.bind "path", width = 400}