If you want, I can sell you the OEM cord for this cheap. I originally (a couple of months ago) added the engine block heater. Then I found out that I had the factory wiring in place I replaced the block heater only cord with the one that included the Y in the cord to plug in the HV battery heater power supply. So I had to replace it.
I'll sell you this OEM block heater only cord for for $20 s&h included ($45 discounted OEM)
I bought the engine block heater (part number F5RZ*6A051*B HEATER ASSY-about $25-30) and cord and did it myself. See here for my journey down this road.

 

I'm actually most interested (in need of) the battery heater. Can the system be installed with just a battery heater?

 

In short, you do not want to just rig up the hybrid battery heater via custom modifications if you are not experienced with high voltage.
All 05-07 and a few early 08 FEH have the hybrid battery heater wiring in place. This means there is a connection from the hybrid battery-thru the DC/DC converter-to the "y" connector under the window washer fluid. All 05-07 FEH sold in the desginated northern states and those with option 41H (engine block heater) have the ability to utilize the hybrid battery heater. In your case you just need option 41H

Tip: use this with an adjustable timer. Set it to turn on 90 minutes before leaving when the temperatures are in the 30's. Increase that to just over 2 hours when the temperatures dip into the mid 20's. Longer for colder weather.