A couple of options:
1) DSL is so small that there's mostly no need to remove any apps to strip it down further. You would just need to install the
mplayer.unc extension and add a line to the
.xinitrc file.
I'm not an expert on the best syntax for the
.xinitrc file, but I
think something like this would get mplayer to run your movie in fullscreen automatically when X starts:
mplayer -fs filename &>/dev/null &. You would add this line underneath the line that runs dillo at boot.
Another idea: If you put a # in front of the torsmo line in the
.xinitrc file, and you use the
minimal cheat code at boot to use the minimal fluxbox window manager instead of JWM, you would reduce the amount of crud on the desktop and speed things up a wee bit. I'm almost certain there's a way to get mplayer to play in the root window (ie, the desktop background), but I couldn't find the command in mplayer's very extensive man pages.
Now, keep in mind: DSL still might not be the best distro for this sort of project, because it uses Xvesa which cannot switch to true fullscreen. If you use DSL you're still going to get the jwm or fluxbox menubar at the bottom, and (unless you can find the command to get mplayer to play in the root window) your video will be playing in it's own window, which might be a little ugly. Also, this seems to me to be a
lot of work just to trick DSL into doing what you want it to do, with a result that might not be QUITE good enough for your needs. So...
2) Another distro you might want to try is eMoviX (
http://movix.sourceforge.net/), which from what I can tell is designed to be small enough that you can burn the OS and a movie file to a CD-R, and the movie will play at boot. I haven't tried it out, but it looks like the sort of thing that would suit your project. It appears to be much smaller than DSL since it's designed with a single application in mind.
3) There are more distros designed with home entertainment in mind at the LiveCD list website:
http://www.livecdlist.com