Alternatives to the on-screen OSD_CAT ?
 

that would be in the packages provided by most distros? OSD_Cat is nice but bit buggy, and slow sometimes...

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.