thanks Jeffrey!
~W

Tim Armes’ LR/Mogrify is likely exactly what you want. —Jeffrey

There’s a 10-image limit only if you’re too cheap to donate the one-cent minimum required for Tim’s unrestricted version. —Jeffrey

If you want only to add watermark during export you may use ImageMagick’s Morgify directly –
just use a free Jeffrey’s ‘Run Any Command’ piglet.

First, download and install ImageMagick
suite, then feed ‘Run Any Command’ piglet with a command like this:

"c:\path\to\morgify.exe" -draw 'image Overlay 0,0 0,0
"d:\path\to\watermark.png"' -gravity SouthWest "{FILE}"

The above example command draws ‘watermark.png’ over the exported
image in ‘overlay’ blending mode and in its lower left part. For
details on positioning and blending mode check - gravity
and - draw options in ImageMagick’s
Command-line Options.

This way you can achieve every transformation provided by ImageMagick
tool. Although it’s not so elastic and easy to use as LR/Morgify
plugin, but there are no limits. This might be a handy way to apply a
common set of transformations.