Ah sorry, that wasn’t clear from your first post. Well, that’s strange. It should have worked then. And since you’re running as root on the monitored system itself, that rules out access rights as well.
Just out of curiosity, what does this do:
cd /usr/lib/check_mk_agent/plugins/
export MK_CONFDIR=/etc/check_mk/
./mk_logwatch.py
It should display at least the logfiles you want to monitor and all new error messages contained therein. And then, add another line to the logfile and see if that gets printed. At least this should confirm your plugin is working as it should.