Directly though the UBP-X800, I suspect it is not possible. But you can use your AVR to force a 24Hz. You won't like the heavy jitters, though.
It depends on which AVR you are talking about...
Personally, I am not too knowledgeable on this topic, but if your display device is a multiple of 60Hz panel, which I believe it is, you are better off keeping the 60hz native. The North American displays do a rigorous 3:2 pull down to convert 24 hz movies to 30 bit and then multiply to reach 60 or 120 hz. A feed like YouTube, which is 60 hz native, when converted to 24 Hz, and then reconverted to 60 Hz for the display compatibility does not usually yield good results.
The problem is with the Epson 5040, which is what I now own. It has a hardware limitation which limits the HDMI input to 10.7 g/b/s - it should be 18 g/b/s...
As a result, when I watch You Tube, which defaults to 60 hz, I lose HDR due to the need for a higher bandwidth HDMI input on the 5040. I could
get HDR, but only if You Tube were limited to 24 hz (maybe 30 hz, I don't know). So I have 2 choices:
1. Watch You Tube videos at 60 hz SDR
2. Watch You Tube videos at 24 hz HDR
but I can't have both 60 hz and
HDR at the same time. The 60 hz SDR looks fantastic, but I just wanted to compare it to 24 hz HDR to see which is the better compromise, and I was hoping that the X800 could be locked at 24 hz.