Data transfer through optical fibre

from your description of the way this is set up, you're probably looking at a solution that is more software related, than it is hardware. since the opto link is already in use, it's a matter of connecting your little boxes to the terminal equipment already in use, and including their data in the already existing data stream.

Serial over Ethernet isn't a bad thing and the suggested parts looks like it could work. The device I linked too even supported TTL RS232.

I'm also not sure if this has to be a single box to box system or multiple (box to box) or a box (master) to many boxes (slaves) or even multiple (box to box) where they can talk to each other.

But, Is the link Ethenet?