Thanks Jacky, Sounds like an interesting plugin for someone to experiment with. I do not think though that we will make this a core feature, but core can certainly help facilitate the plugin.
If you have budget I recommend posting in Marketplace, my guess is that a minimal prototype plugin (with minimal configuration - only supply text) would take about 1 to 2 days to build.
Watermark by superimposing images may take a bit longer to get right.