This will largely depend upon the chipset and drivers supporting passthru bitstream, and of course the App. This is already possible with some, but not all, Android TV Hdmi 2.0 boxes with the SMTP version of Kodi. Some required a special firmware and others work just fine with the vanilla version of Kodi. I suspect the Plex apps maybe similar.
Since the intended purpose of eARC is to pass the full range of codecs in theory it will, in practice however it will be a very different story. It may be a bit hit or miss when capable TVs first arrive. Likely the big brands such as Sony, LG and Samsung it won't be a problem. I do forsee a lot of threads when this turns up.