For implementing arrow annotations in djvu.el, I have extended svg.el with markers/arrowheads.
It is just a little extra code (and some documentation in the preliminary comment). You can check out the difference by diffing the following
svg2 (I simply used that name, it is not really a 2.0 version) with the original svg.el.
Of course I am willing to add documentation, but learning all aspects of Emacs development takes so much time that this will probably take some time. If someone likes it and is able to assist then it would be great, but if not then that is totally fine too...
The main reason for this message is to at least inform you about it.
Thank you!