Currently we have multiple ways to modify environment (variables, working directory, limits etc.) for a service without touching its startup script but no way to change default umask(1) value. None of tools working with login classes change umask.
Also, we have no ways to extract umask by class name for shell script. Let us deal with the problem in simple way via new servicename_umask knob for rc.conf(5)
Some file-sharing services that create new files may benefit from it.