I only had about 30 minutes to look at this, but here's a bit of a workaround. It will require having one channel that you manually "turn off." I used my sub channel and just turned off my sub amp. Anyway, hope this helps.
Open the Audio MIDI Setup utility
Click on the plus, add an aggregate device, then select HDMI. This will give you access to all the channels, but you only want to activate one at a time. Click the "Configure Speakers" button.
In the resulting window, make sure the "Stereo" Button is depressed. This will let you select the channel you want, and you can select from any of the channels available to your AVR. We need to manually turn one of these off, so as I mentioned, I set the "Right Front" to my sub channel, and then turned off my sub amp. Now you can use the "Left Front" to select whichever channel you want. I ran through all the speakers in my setup. Seemed to work.
You'll need to make sure you select the aggregate device in REW. Whenever you want to change channels, you'll need to go to the MIDI Setup utility and select the channel you want. Not much different than Soundflower.
That's the best I can do for now.
EDIT: BTW, I ran a few sweeps and had no pops or buzzing.