I have a WAE-674 connected to two 6509 switches (sup720). The two WAE ports are in a standby group. I'm also using WCCP L2 redirection from 6509 to the WAE, but I remember in older WAAS software, WAE standby group was not compatible with L2 redirection mask-assignment. Only hash could be used.
Is it still the same with the newer 4.2.1 software? I couldn't find specifics in its documentations.
I believe you are talking about this defect: CSCso66693
Now, Standby interface does work with L2 Masking.
Details about virtual standby interface:
The virtual standby interface uses the MAC address of the active interface. Standby interfaces remain unused unless an active interface fails. When an active network interface fails (because of cable trouble, Layer 2 switch failure, or other failure), and that interface is part of a standby group, a standby interface can start to carry traffic and take the load off the failed interface. With standby interface configuration, only one interface is in use at a given time.
CLI command: show interface standby 1
Please note that VB interface bridging is not supported when you are using standby interface feature.
Hope this helps.
PS: If this answers your question, please mark it as Answered.