I have devices that send out syslog from different interfaces which they are managed. Currently, libre does not support syslogs from these devices. This would also be useful if syslogs were coming from an intermediary device (log collector) instead of the device itself.
LibreNMS is able to connect received syslog messages to a monitored host, if the message is correctly tagged with the hostname exacly how LibreNMS knows it. (That is, if LibreNMS knows the host by FQDN, the syslog message must contain the FQDN. And vice versa.)
I would suggest putting the burden of getting a correctly formatted syslog message on the devices in question, to the extent possible. Syslog.md and Graylog.md have some advice in this regard.
LibreNMS is not a syslog receiver, but may make use of a local syslog-ng/rsyslog for the purpose of getting at the received messages.These tools are specificly made for the purpose of handling syslog messages, which LibreNMS is not. One must assume both of these tools can be set up for modifying the received messages prior to passing them on to librenms.
How to do this is really outside the scope of LibreNMS, but is certainly of interest to users of LibreNMS.
If you find a way to configure your devices to properly format the syslog messages, it would be best. If not, see if you can do it by means of configuring syslog-ng/rsyslog. In either case, please document it and update Syslog.md accordingly.