Well it’s not convenient but if you’re willing to use a complex ffmpeg command-line invocation, you should be able to re-project one lens from the stereoscopic view into a flat view.
I’ve used a (more complex than you’d need) command to create 3D VR180 videos that can be viewed in YouTube, both in 3D in VR and in 2d on screen through the web site or mobile app:
You could cut down that command to take only one eye/lens view, and project that to a “flat” projection cropped to a limited field of view.
There’s probably a simpler way though. 