I want to force the X server to use a DPI of 96 (this is for an openSUSE
system inside a virtual machine; I'm not forcing DPI on my real
installation).

I tried to add the "-dpi 96" option to the ServerArgsLocal entry in
/opt/kde3/share/config/kdm/kdmrc file from the default:

ServerArgsLocal=-nolisten tcp

to:

ServerArgsLocal=-nolisten tcp -dpi 96

However, it simply doesn't work. The option isn't passed to the X
server. DPI remains unchanged and, according to ps, there no sign of
"-dpi 96" in the arguments of /usr/bin/Xorg.

What am I doing wrong?

PS:
Note that simply forcing a DPI of 96 in the KDE control center *does*
work. But I want to set it globally for the whole server, not on a
KDE-only, per-user basis.