Yeah, I was kind of the mentality of "Well, if its HDMi, its basically single link DVI..." Of course, it is its own standard, CEC is probably the simplest way of achieving this, there is a CEC-serial bridge, various websites have them. I've looked into them before, before I started using all Macs because of my job. The old Pi sees little use these days =(
The thing with the CEC-serial bridge is, do I spend $x on this device, to save $y worth of power over z period of time, and at the end of the day, am I making an acceptable ROI? YMMV. I just had this same debate with a bloke from the Czech Republic about the Australian Government incentive scheme for household solar panels, and no energy storage facilities. Again, YMMV.