### unsupported bla bla bla don't want to hear it ###
ok here is my advice on how to hack this to do what you want. No idea if this will work on all scenarios but there is a chance this will hold even during upgrades.
Come up with a custom rsyslog.conf file.
Store it in /pfrm2.0/etc
edit /pfrm2.0/etc/userScript (not created by default perms should be root:root 755)
have userScript file copy /pfrm2.0/etc/rsyslog.conf to /etc
#I'm saying do this because / (and thus /etc) is a rootfs which is basically a memory based filesystem.
have userScript restart rsyslogd. You'll have to find pid kill it and start it.
Now in theory it will get reinstalled on boot up.
Last step is edit /pfrm2.0/etc/additional_settings_file_list
and add /pfrm2.0/etc/rsyslog.conf. This *should* protect you from upgrades.
edge cases this won't help with..
someone editing syslog settings via clish or webui. Most likely your settings will be lost until you rerun userScript commands or reboot. Possible other changes in clish/webui might trigger reloading build rsyslog.conf.