run an OpenVPN server from my router as well
Why do you need openvpn on OMV if you have it on your router?
Possibly a stupid question, but I have to ask: can you run shell scripts from Jinja/Salt?
Probably but that is a terrible way to code a plugin.
I tried answering this myself by Googling "jinja/salt execute shell script", but I cannot find a straightforward answer
Salt (really saltstack) is a configuration management engine and it executes the command Jinja is the template language.
{% set load_avg = salt['cmd.shell']('uptime | sed "s/.*load average: //" | cut -d " " -f2 | cut -d . -f1') %}