Few thoughts. Just throwing stuff out to get the creative juices flowing.
Here is how I think it looks - could be wrong, but hopefully close enough that we can talk in general terms.
Ok, so here are my thoughts.....
You could track down a Ethernet to serial device. I have used these in the way distant past. They are a thing.
You need two, one at each end of the IT fiber.
You could do what I have done in red, that is, drop an S controller in the substation and run the B100's there off one com port and run the other remote B100's off another com port.
Not shown, but you could drop an R in the substation and move the existing digital points over to SNAP digital modules and then write a chart to bit bang the remote B100's. (Personally I am not a good enough programmer to pull this off, but suspect it's doable).
Yes, driving remote I/O from a SNAP rack serial module would be cool, but there are many deep reasons why we are not likely to see this, so I would not hold my breath while I wait for it to be a solution.