I figured out a way that looks like it should be possible, but i havent tested:
in /system>media, there is a zip file called bootanimation.zip. inside are a whole lot of images, which going through quickly or in a fast slide show makes the bootanimation. my thought is that if one were to edit those pictures, say using paint, then the bootanimatin would now include your personal message. you would have to make the exact same edit in each picture, but i think it might work. anyone willing to try it out? if you do, please post your results!