It is possible to 'trick' the teltonika software to enable this mosquitto feature by manipulating the qos setting in /etc/config/mosquitto via ssh.
option direction 'out'
option qos '0 "" test_prefix/'
option topic '#'
Notice the ' quotes that allow the template of the real mosquito config to be set correctly. This enables us to deploy identical software without configuration to all our target systems and have only the router configuration differ by adding different prefixes.
A native implementation via the webinterface would be welcome though.