If you're thinking [empeg usb] -> [usb<->serial mulitplexer] then the following may have a bearing:

The empeg USB port is not a USB host, only a device. I'm not a USB expert, but I believe that means it cannot initiate communication. That'd mean that we'd need a hijack hack to implement a new device type for the mulitplexer to speak to, and the multiplexer would have to handle intiation etc.
It would also require a physical sled hack to add a USB connector. (But so would an ethernet device).

If you're thinking:
[empeg-serial] -> [serial <-> USB multiplexer]
then we'd then need serial-usb converters for all serial devices. There would also be a bandwidth issue.

But if we're talking about implementing an external hardware device to do this, maybe we should consider a system on chip board. Overkill for just serial ports, but it would open up a lot of possibilities for e.g. additional display - broadcast the current display via UDP.