GTAC got me straightened out. I think I was running into a wall trying to change through the various settings without downing the ports in the process. GTAC tells me that the correct mode for the channel-group is "on". The Wireless LAG ports do *NOT* use LACP, and so this needed to be configured as a "static lag".
Here is that article.
And here is the config that worked!
interface Port-channel2 switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet1/0/51
description EXTREME WIRELESS CONTROLLER LAG2
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 2 mode on
!
interface GigabitEthernet1/0/52
description EXTREME WIRELESS CONTROLLER LAG2
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 2 mode on
!