Are these connection IPs hard-coded into the source?
No. You connect to an IRC channel to get a list of all active Bitcoin peers. Then you choose 8 at random (basically). You will also be listed on that channel, though peers won't be able to connect to you from the outside.
I recommend running the miners with -connect=<IP of networked bitcoind>. This connects only to the specified IP, and it prevents you from being listed on IRC. Then you are well-connected, but you avoid wasting network resources.
Not being well-connected is only a minor disadvantage. You can replicate some of the advantage by running the miners with -maxconnections=20, which will increase the number of outgoing connections to the specified amount. You might also do -maxconnections=4 and -noirc to decrease the number and stay off IRC (you'll bootstrap from your network node), and then use addnode to connect to your networked bitcoind: this decreases network load without relying completely on your networked computer.
(Edit: I just discovered that maxconnections can't increase the number of connections from 8, as I always assumed.)