How to transmit HDMI data over fiber
Could you change the transport layer of the fibre to be ethernet? 

That way for example you use "just add power"'s HDMI over IP, it would mean the existing data steams would have to be migrated as well, but there as are solutions to convert almost anything to ip, you could fix the existing issue and be ready for whatever else in future want to be transported over it.

You could then also rely on off the shelf networking equipment and protocols to manage priority's, time slots, redundancy etc. (and to provide spares!)