Você deve ser capaz de executar um "shell execute" que chame um script que modifique a gravidade e use logger
para enviar de volta.
De man rsyslog.conf
Shell execute
This executes a program in a subshell. The program is passed the template-generated message as the only command line parameter. Rsyslog waits until the program terminates and only then contin‐ ues to run.Example: ^program-to-execute;template
The program-to-execute can be any valid executable. It receives the template string as a single parameter (argv1).