Some receivers will not send digital sources to zone 2.
Check the owners manual.
An external bluetooth receiver connected to an analog input would give you BT in zone 2.
Add an analog output from any sources you can. Assign that connection to a different input name than the digital connection so you can access it for zone 2.
Just to confirm... the receiver is a Yamaha RX-V685?
If so, then it's 100% possible.
You can actually use MusicCast to share any source from Zone 1 to Zone 2 (except for Airplay and DSD audio files.
I would hazard a guess that you have the 'power amp assign' setup incorrectly and aren't actually sending audio to the Zone 2 speakers, instead you're just playing radio through the 'surround rear' speakers when in a 7-channel stereo setting (or similar).
Step 1 : Make sure you setup the Power Amp Assign correctly. You can use the receivers on-screen GUI to do this.
Step 2: Download the MusicCast app and make sure the product is linked as per the app's instructions.
Step 3: On the MusicCast app, turn on 'Zone 2' in the settings. A separate room will then show up that shows you the sources that can be independently selected in Zone 2. This makes it super simple to see what is possible.
Step 4: Start playing around with the 'link button' and see what you can share from zone to zone... the beauty with MusicCast is that you can share HDMI / optical / RCA / coax / USB / DLAN / Spotify / Tidal etc etc... so most of your needs should be sorted.