HI , i'm not expert with pulseaudio framework but i think this could be possible,,,
1. HDMI output to TV configured as center speaker/channel.
2. Analog/Spdif onboard audio out as Front L/R channels virtually 2.1 depend on speakers setup
3. Bluetóoth A2DP Sink as Rear L/R channels
This will create 5.0/5.1 speaker configuration.
I don't know how to write config for pulseaudio and i don't know if this is be accepted by Libreelec subsystems and don't provide errors in menu/settings/audio. 
Hi, i'm still thinking this throught.
But i think you would have to load your sinks in PulseAudio.
Example: pactl load-module module-alsa-sink "etc" For every output, and respective params (channels, map, alsa card, output name)
Then combine the sinks.
pactl load-module module-combine-sink "etc" (output names, map, channels)
Haven't tried anything like it though.
Depending on the Bluetooth latency, this may sound pretty poor. You could easily have the speakers linked via Bluetooth playing their content with a noticeable delay - could be up to a few hundred milliseconds. There are low latency Bluetooth codecs that may help. Having said that, in my view, the effort needed to manage this level of complexity would be hard to justify.
Depending on the Bluetooth latency, this may sound pretty poor.
I think this delay to rear speakers conected trought bluetooth in hundred milliseconds could be customizable by individual user needs , sended little time before video stream with rest audio appearing on screen, I think some additional muxing in maybe "player core ?" with cache/buffer could be the way to realize that...
You can/could bastardize something in pulse to do this, but it will be complicated as hell to configure and prone to timing issues so I just wouldn't bother. Save pennies and get a proper HDMI 5.1 surround speaker arrangement. The result will sound better and be zero effort.
Hi. maybe author of this addon have better knowledge and wan't to implement "combined audio output"
https://github.com/Heckie75/pasink
