Não testei, mas acho que você precisa de algo assim:
<Extension multi>
Module xm_multiline
HeaderLine /^================/
EndLine /^===============/
</Extension>
<Input in>
Module im_file
File "input.log"
InputType multi
# Remove the boundary markers
Exec if $raw_event =~ s/========[=]+//g {}
# Make a single line
Exec $raw_event = replace($raw_event, "\r\n", " ");
</Input>
<Output>
Module om_udp
Host 1.2.3.4
Port 514
Exec to_syslog_bsd();
</Output>
<Route>
....