I know RPi have added support recently (https://github.com/raspberrypi/firmware/issues/683#issuecomment-283179792) - so I was wondering if something similar might have made it's way into kernel to support a similar approach to outputting 240p NTSC/PAL.
I think it might depend on if the A/V chipset can support whatever trick is required to support doing 240p from whatever is fed to it. I'm not sure where'd I start looking for details on that.