Here you wil find a modified version of xmatrix, one of the hacks from jwz's XScreenSaver. Instead of drawing to the screen, it writes each frame to a file in XPM format.
To build this, get a copy of XScreenSaver, replace xmatrix.c with this one, and build as normal. You probably don't want to install it system-wide though, because this would be an awful screensaver.
I hacked this together in a few hours, in order to make a boot animation fon an Android device (whose display has far more pixels than my computer's), and I thought it might be of use to someone.
Although the original works on OSX, this probably doesn't yet. Patches are welcome.