Does Screensurfer support multiple SNA gateways?  The 
context is where a group of SNA servers have been set up to 
achieve resilience by automatically rerouting client 
requests from a gateway that has gone down to one that is 
still running.  Will Screensurfer data be successfully 
rerouted?

This is actually a function of the SNA Server setup.

When an SNA Server connection is defined in Screensurfer, 
you simply give the name of the SNA Server "pool" you will 
be connecting to.

It actually is the SNA Server Client component executing on 
the Screensurfer server that determines where to obtain and 
LU session from the named pool.

While I have understood that it is possible to setup 
failover within an SNA Server cluster, I don't know how to 
specify it...

In Screensurfer itself, you can use the *ANY connection 
type, try to connect to one pool name and if it's down, try 
the next, but I think that the native SNA Server support 
for failover would be a better approach.