In bridge-mode you will loose a lot of functionality and flexibility. Beter use redundant controllers. You have a Mobility Master so you can use clustering for high availiablity, seemless failover and live-upgrades.
ArubaOS 8.4 User Guide Page 1136 (link)
In the ssid virtual AP profile you can set a "operating mode" value to backup or always. When configured as always the SSID stays up while the WAN Uplink is down, but this not true for a CAP that is managent by a controller, it still need to exchange management packets with the controller.
It could be worked when you configured a CAP as RAP (Remote Access Point). But my advise is to keep using tunnel-mode and make use of redunant controllers in a cluster.
For references see the RAP support modes.