The BD-Live part is easy - all you need is that single Cat5 (or Cat5E or Cat6) cable.

The Gefen adapter you found will certainly take care of the iMac audio, and I assume you have some solution in place (either the iMac's IR remote or something like Apple Remote on an iPhone or iPod Touch) so it is convenient for use when setup remotely like that.

The video side can be done with two Cat5e runs and a device like this . Gefen also makes one .