An update.
I got my DroidTV MX with the GBox Midnight MX2 firmware on it. Works fine out the box, XBMC works although I havent figured out filesharing yet. That is, how to share files shared on a windows share with my Android PC. I'll get to that.
As for synchronized music playback, its working! I installed XSqueeze on the Android box, but Squeezelite is not Android compatible. So, it can function in controller only mode - no local playback. However, there is an app in the store that is an LMS player, which I paid for (R50) and downloaded. It works! This is a much cheaper way to get synchronized music playback to work.
With one caveat, which is that I dont have a DHCP server on my network - I didnt think about it. So, my Android box and my PC are connected to the same wireless network, but I had to manually specify the PC's IP address when configuring the Android box. Come to think of it, this would affect filesharing too. Anyway, what I might do is go into my router's configuration page and reserve an IP address for my PC so that it doesnt change.