I think srsa_4c may have explored most of this already, but anyway:
The canon code references displaytype values 11 and 12, with resolutions 1024x768 and 900x600.
On sx710 (101a), these can sometimes actually be set, using the function called by DispCon_SetDisplayType:
=call_func_ptr(0xfc0f0dfb,12)
The canon UI is garbled, but CHDK OSD is OK if the buffer sizes from canon code are used. I think it crashes if the function is called with an image on the camera, but there might be random crashes too. Switching to rec crashes if PTP is connected, but works if not. The live view is garbled in rec. Shooting works.
I don't think there's any real use for these modes.
I also found that if you connect HDMI to a lower res monitor (1280x1024, with an HDMI-DVI adapter in this case) it uses displaytype 8, 720x480 res. This low res HDMI still doesn't allow rec mode. Unlikely other TV out modes, this is 8 regardless of the NTSC/PAL setting.
In regular HD HDMI, the playback live buffer is full 1920x1080, 2x the size of the bitmap. The code I used on G7X and sx710 supports this over PTP, although I have trouble imagining a use case for it.
