Of course the POP3 switch described is just a TCP/IP data pipe and hence is extensible to just about any protocol that uses TCP/IP. The only penalty is that the further down the list of possible host/port pairs the switch has to go before making a connection the longer the connection time becomes. However some sort of caching mechanism by which a bad host/port is not tried again for a time could improve this.
Hence we are able to swap in backup servers using ARP spoofing and have them point to content where it is still available using TCP/IP data pipes.