Following up on this. I tested out my M2 Max Macbook Pro and M3 Max Macbook Pro with three 4k 144hz (28” Odyssey G70B) monitors. Two are plugged in via DisplayPort 1.4 and the third via HDMI 2.1. If I drop the refresh rate to 120hz I can enable HDR on all three. I'm also seeing that using scaled resolutions works at 144hz just fine even though internally macOS is doubling the resolution (6720x3780).
Everything works just the same on both M2 and M3. I did have some issues with the monitor plugged in via HDMI, though. If I didn't have it properly set to PC mode in the monitor's onboard settings it would cap out at 120hz instead of 144hz.
System report screenshots: