upstream: ip_hash and backup

[John Moore]

I'm using the upstream module to load balance between 2 back-end 
servers, using ip_hash to effect a kind of 'stickiness' (making it more 
likely that subsequent requests from the same client will go to the same 
back-end server). Now I want to add a 3rd back-end server into the mix, 
but which will only come into play if the other 2 are failing. There's a 
handy 'backup' parameter I can use for this, except that according to 
the docs it cannot be combined with the ip_hash directive. I'm wondering 
whether there is some other way to achieve what I want here? 
Effectively, the ip_hash becomes irrelevant once it's failed over to 
this backup server as there is only one.