I’m not familiar with ZDMC, but I highly doubt you can bitstream AAC. And you CAN’T pass through multichannel PCM, because ARC only has about 1.5Mbit/s of bandwidth, which is enough for PCM Stereo or DD5.1/DTS5.1.
You can select the HDMI audio output, set it to 2.0 channels, enable passthrough and enable DD Transcoding to AC3. That way it should work for what you want to achieve.