If there are multiple SLiM services, it doesn't work, because "[t]here
can be only one instance of an extensible service type" (according to
'Service Types and Services' in the manual).

Because of that I believe making slim-service-type extensible would be
more confusing than useful, which is why I didn't pursue this further.
Maybe it should be mentioned in the manual that 'set-xorg-configuration'
doesn't work with SLiM?

Regards,

Diego