The attached patch will allow logwatch to parse systemd journal entries. It checks the logfile config for a "Journal" entry and uses that as a filter to journalctl. As an example, a user can switch the postfix service from using /var/log/maillog to the journal by creating a /etc/logwatch/conf/logfiles/maillog.conf with the following:
Journal = "_SYSTEMD_UNIT=postfix.service"
I've tried to keep the changes to a minimum, but I've also included a separate patch that makes testing a bit easier. It looks for the LOGWATCH_TEST env variable. If it exists, it expects to find all the config/etc files under that directory. Of course, the journal patch doesn't depend on the testing patch.
I'm happy to do additional work on this if it is well received. It could certainly be more efficient, and it needs to be documented.