We promote redundancy via diversity. With this we mean that the complete path must be redundant. A failover relay isnt more reliable than the amplifier, so such a scheme is not providing full security. 

So, the short answer is no, but we really think that diversity with an A system and a B system is the way to go. It has been used on many profession sports venues in Europe and the concept is that every 2nd loudspeaker belongs to the A system and the other ones belong to the B system.

If anything fails in system A, then there will be enough leakage from system B to cover enough of the audience to make the others understand whats going on. This is certainly accepted from a compliance standpoint for voice evacuation.