iRule to select test pool based on cookie/env variable?

You could add the new member at a lower priority and then use an iRule like this to allow manual selection of it with a query string parameter and cookie:

http://devcentral.f5.com/wiki/iRules.Select_pool_member_based_on_HTTP_query_string_parameter.ashx

When you're done testing, change the priority of the new pool member to the same as the other one to have it used in normal load balancing.

Or as Brian said, if you can filter by client IP, it would be simpler to do something like this:

when CLIENT_ACCEPTED {

   if {[IP::addr [IP::client_addr] equals 1.1.1.1]}{
      node 10.1.1.1 80
   }
}

With this simple iRule you don't even need to add the new member to the pool. When you're done testing, just remove the iRule and add the new member to the pool.

Aaron