In my experience, the first place you should look when searching for alternatives is usually your package manager. In this case, a quick apt package search turned up aosd-cat and gnome-osd.
After a few minutes of playing around with them, aosd is very similar to xosd and therefore easier to adapt to an existing script than the xml-based gnome-osd. However, for some reason I can't get true compositing transparency with aosd, although both xosd and gnome-osd work just fine. YMMV, of course.
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