I checked but the only thing in /usr/share/mplayer/ is the skins/ directory.
I did manage to stumble across a file called /etc/mplayer/mplayer.conf. That one is entirely commented out except for also a few lines about fonts.
fontconfig = yes
font = "Sans"
subfont-text-scale = 3
One of the things it says is
# MPlayer configuration file
# Configuration files are read system-wide from /usr/local/etc/mplayer.conf
# and per user from ~/.mplayer/config, where per-user settings override
# system-wide settings, all of which are overrriden by the command line.
so, at least according to that, ~/.mplayer/config is the relevant config file.
Oh well, thanks again. Maybe I'll look again through the different runtime options that can be passed.