Comment # 9 on bug 935559 from Fabian Vogt

SDDM has the ServerArguments option in /etc/sddm.conf for exactly this purpose.
The issue is that /etc/sddm.conf is currently part of the sddm rpm itself, you
changing it invalidates the package installation (you'll get .rpmnew files if
the config file in the package changes) so it's not really ideal.

Adding a sysconfig variable is IMO a bad idea as:

a) sysconfig/displaymanager will likely be deprecated soon
b) How to handle it if both are defined?

There is luckily work ongoing upstream to support /usr/share/sddm.conf.d/ and
/etc/sddm.conf.d directories which makes sddm config customization much more
sane and is likely what we should go with.

So closing as WONTFIX (if you really really need this, please reopen and the
pending upstream changes for the conf.d support can probably be backported).