It's not much but any interest in putting this or a variation into the code?
Change email subject from:
[Bro] Connection summary from ${TIMEFRAME}
to
[Bro] Connection summary from ${TIMEFRAME} - ${HOSTNAME}
# diff ${BROINSTALLPATH}share/broctl/scripts/postprocessors/summarize-connections ${BROINSTALLPATH}/share/broctl/scripts/postprocessors/summarize-connections.orig
25d24
< systemname="$(hostname)"
82c81
< "${scriptsdir}"/send-mail "$subject - $systemname" <$output