So I played arround with the liveview in MV1080 and stumbled uppon this little trick:
When in MV1080 and using Realtime Liveview you get a more accurate, fluid representation by using "Anamorphic 9:5" and "DigitalPanning".
However the framing, although better, is still not properly centered and is missing info on the top and bottom.
Not sure if this helps, but maybe there is a way to fix the liveview this way?
Although Im optimistic there is a way to make it work properly... at least in 16:9... but I wouldnt know where to start.
Here some comparisons:

Streched Realtime liveview

Correct framing but slow liveview

Anamorphic9:5 and DigitalPanning Method

Recording is not affected

Settings
...
Hope this is helpfull in any way.
Cheers!