as it is said on this page http://mplayerplug-in.sourceforge.net/config.php
mplayer-plugin can be configured in one of those 3 files
$HOME/.mplayer/mplayerplug-in.conf
$HOME/.mozilla/mplayerplug-in.conf
/etc/mplayerplug-in.conf

you could give an example of such a config file in the doc directory ... oops arch has better decided not to give the user any doc ...

so the last option is to put a /etc/mplayerplug-in.conf in the package like this one below (created from the web page cited above)
-----------------------------------------------------
#Option Name=[values] Description
#Enable debugging if set to 1, very verbose debugging set to 2
#debug=[0|1|2]
#Note: value is in Kilobytes. Default value is 512
#cachesize=[0 - 65535]
#If used with the download option. This is the buffer size before the video plays.
#Alias for cachesize option
#cachemin=[0 - 65535]
#Percent of file to load into cache. Default value of 25
#cache-percent=[0-100]
cache-percent=0
#For media larger than 16K it is played directly, useful for diskless terminals
#nomediacache=[0|1]
#Set QT Speeed, use low for dialup, medium for DSL, high for more
#qt-speed=[low|medium|high]
#Overrides mplayer default config values.
#vo=[xv,x11....etc]
#Overrides mplayer default config values
#ao=[arts,esd,oss...etc]
#Note forces mplayer to be in it's own window if set to 1
#noembed=[0|1]
#If set to 1, mplayer will be launched in its own window if the url is not in an EMBED tag
#fileonly-embed=[0|1]
#When set to 1 uses the aspect of movie no matter what
#prefer-aspect=[0|1]
#Pass special video post processing options to mplayer
#vopopt=[mplayer vop options]
#Tells mplayer to allow default video post processing or not
#novop=[0|1]
#Set the OSD mode
#osdlevel=[0|1|2|3]
#Option for rtsp streams, set to 1 when behind proxy
#rtsp-use-tcp=[0|1]
#Passes display to show output to, useful for multiple monitor setups
#display=[display name]
#Where to store the downloaded movies
#dload-dir=[writable directory]
#Example: $HOME/Movies
#Set to 1 if you want to keep downloaded media
#keep-download=[0|1]
#Read mime types to handle from [ $HOME/.mplayer | $HOME/.mozilla | /etc ]/mplayerplug-in.types *
#use-mimetypes=[0|1]
#Enable Real Media Support *
#enable-real=[0|1]
#Enable Windows Media Support *
#enable-wm=[0|1]
#Enable Quicktime Media Support *
#enable-qt=[0|1]
#Enable MPEG Support *
#enable-mpeg=[0|1]
#Enable Ogg Vorbis Media Support
#enable-ogg=[0|1]