What if you choose channel 1 under both left front and left surround, and channel 2 under both right front and right surround?
Edit: The above does not work. Maybe I found a solution but I cannot test yet since my 8-channel DAC did not yet arrive.
Solution: install Apple's AU Lab and add a new configuration (it's the "+" button at the bottom) under user configuration, where you can define input and output channels. You have a matrix mixer to route channels as you wish.