I run a second zone to my family room and my patio. Since they are both on the same zone, the volume scales together. They are in parallel, and don't seem to present a difficult load to the old receiver that powers them. I don't generally use them at high volume, so the lower impedance is less of a problem than if I were to play loudly.
I use an RF remote and I'm not sure how I could live without it. Good choice if you're going that way.
I use an in-wall volume control, probably similar to the ones you're considering. This is run to the speakers on the patio only. I'm therefore able to reduce their volume with respect to the ones that are in parallel with them. Of course, for there to be volume on the patio, there must also be volume in the family room (the other half of "zone 2"). This works just fine for us.
I would not worry about the volume control sucking out your power. FWIW, I'm using a passive volume control from Radio Shack, and have never had a problem with volume when I wanted it.
How loud do you think you'll play this second zone?