Why to bother with all this?
I use (successfuly) the same method as on my debian-box.
I've put Doom2.wad file on MMC and then went to
/usr/share/lx-doom/wad
su-ed
renamed oryginal wad (I didn't want to remove right now):
mv doom1.wad doom1.-wad
created symlink to mmc:
ln -s /media/mmc2/DOOM2.WAD ./doom2.wad

voila
My all id games on my PC work that way for their "big" PAK's.