You could first try just grounding one side and feeding signal into the other; this should work. Try with a low volume first. You will loose the extra noise immunity that differential gives you, but I suspect you won't have many problems - the main reason to use differential on a changer bus is to stop noise pick up without using any shielded cables.

Ford's ACP changer bus uses a similar system, as 600 ohm differential. The car outputs are 100 ohm, so there will be no issue driving the load.

Hugo