Have not personally done Cobranet but looking over the documentation this all seems to indicate the Cobranet requires all the devices be in the same VLAN.
But it does allow you to connect Cobranet specific repeaters to connect Cobranet devices in different physical areas. Based on the examples here, I would assume you could perform the same function using standard Ethernet trunking to get the VLAN distributed to all the necessary points. But the bottom line is the Cobranet devices need to be in the same broadcast domain in the end.