It's kinda strange that there aren't any cheap automatic Toslink switches available as detecting an active source is quite easy.
In case someone wants to know how the detection works:
When there's no active source connected to the Toslink receivers, then the output level of the receivers will be uncertain. It could be 0V or 5V. (Or even toggle at some point.) But IF there's an active source connected, the receiver's output will toggle all the time.
By using the microcontrollers pin change interrupt capability it's convenient to detect if the Toslink receiver's ouput has been toggled. Just wait a certain time (currently 200 us) and check the appropriate flag. If there really has been a change, a counter gets incremented. Repeat the procedure a number of times (currently 50 iterations). After that the counter is evaluated. If the counter is above a certain threshold (currently 70% => 0.7 * 50 iterations = 35), then this Toslink input is treated as active.