Yesterday at Bryce's prompting I wrote a little effect that makes the selected objects a <marker /> in the <defs /> of the current document. Inkscape doesn't really seem to deal well with this but there is some code there to work with if anyone has ideas on how it could be used. I think Bryce made some modifications to his copy as well. Because there is a lack of interactivity you must position the connection point of the marker at (0,0). And there are likely a few other nuances I haven't thought of yet.
http://www.ekips.org/comp/inkscape/inx/makemarker.tar.gz
I also wrote a python script that embeds images in SVG because I didn't feel comfortable editing the perl script. It functions as an effect and can be called from the effects menu. If objects are selected it will only replace the images in the selection, otherwise it should replace the images in the entire document (although in retrospect I didn't test that, oops). Hopefully this effect will be obsolete soon and we can have "embed all" in a menu and "embed" in the images' context menu.
http://www.ekips.org/comp/inkscape/inx/embedimage.tar.gz
Aaron Spike