What we really need is the same watermark feature that we can position at the bottom of the page, but has a white background to show the lettering better on a consistent basis.
You could create a simple white
png image, then place that image as an
Image Logo in the
Watermark... control panel (in addition to the
Text), to act as a background for your generated watermark text.
Obviously this will cover part of the image, but you could experiment with the shape, colour, opacity and positioning of this 'logo' to render a pleasing result.
A very quick proof of concept using a dark and light image (also rendering other image variables in addition to {caption})...
I wanted to demo a simple but more elegant result but there's either something weird going on with PM's font rendering in the
Watermark... panel, or I'm being mindnumbingly stupid.