You can enable the highlight warning separately in the display custom settings, then it works in video mode too. But unfortunately you are correct about the RGB histogram, there is no way to show that in video mode.
I few weeks ago made a video about F-Log, you might find it helpful:
As you can see you can live without F-Log, and it's not without problems. I guess you would run into even more problems if it was recorded in-camera with 100Mbps H.264.
Exactly, and you don't really want to use F-Log with 8bit 4:2:0, I have found it to be unreliable even in 8bit 4:2:2 Prores. Sometimes even a simple S curve and a gamut conversion is enough to break it, I think it really needs 10bit, or some tweaking. Let's wait and see if the X-H1 gets it externally. My hopes are pretty low though.