by Davo Smith.
I'm not aware of a plugin that supports that and you certainly can't do that with the standard Moodle code.
It should be possible to re-purpose some of the assignment editpdf code in order to achieve what you are wanting, but it would require a developer and quite a bit of time in order to do it (a vague, off the top of my head estimate, would be 3-4 weeks of developer time, but that's without having looked in detail, but with a good understanding of how much longer than that it took me to write the original PDF annotation plugin in the first place).