Resolution:
Q). If a  SS 6.x RTserver sets the following options
       setopt client_read_timeout 30.0
       setopt client_keep_alive_timeout 15.0
and the connected clients are SS 5.5r5 will they respond to the client_keep_alive messages?
(Is there a callback in the 5.5r5 client that responds to a client_keep_alive message from the server)

A). Yes, this feature was actually added in SS 5r5.5 so the support (i.e. callback in the client code) should be there. The client application though will of course need to process the keep alive messages by calling mainloop or whatever. In other words, can't be just a pure publishing client.