I just had this exact situation come up. The client is using a voice
VLAN routed at the FortiGate to the data VLAN. The FortiGate is
configured with the internal1 interface on the data VLAN and an
additional tagged VLAN interface for the voice VLAN. ...
But as it is currently implemented the bridge is between an SSID and the
physical network interface of the FortiAP. The FortiAP does not support
802.1q as far as I am aware.
I believe the limit of one bridged SSID per AP is a technical limitation
rather than a FortiGate limitation. Doesn' t make much sense to have
multiple SSIDs bridged to the same physical network.