Hi @DaltonHybrid, welcome to the forum 
We’re not entirely clear here on what it is that isn’t working. The Bridges service in BlueOS (requires Pirate Mode to see) should be able to forward serial data over UDP, which should be equivalent to the functionalities available on Companion’s Routing page - is that not working for you? If so we’ll need to investigate and fix it.
Assuming Bridges does have some bug or issue, you can potentially try installing @williangalvani’s VirtualHere extension via the BlueOS Terminal page, then install a VirtualHere client on your topside computer, which allows presenting a USB device that’s physically connected to the Raspberry Pi as a USB device connected to your topside computer, which may then be detectable by your sonar software. Be aware that our extensions architecture is not stable yet, so working with any extensions is currently only available via terminal commands, and will have limited support.
If it’s going via the Raspberry Pi then it should be sendable through the same tether pair as the rest of the ROV communications (although there may be bandwidth / performance limitations when doing so). If you want to send an RS-485 signal directly to the surface then that will need to be on a separate pair, to avoid conflicting with the ROV communications, which use a different protocol.