I don't think that HDMI input can go to MT8291.
Here is some description of MT8291 from other TV service manual:
The MT8291 is a highly integrated stereo audio CODEC. The MT8291 performs stereo
analog-to-digital and two digital-to-analog conversions with single-ended analog voltage input
and output. It?s up to 24-bit serial values at sample rates up to 192 kHz. A 7:1 stereo input
multiplexer and an automatic level control are included.
AFAIK HDMI goes to SIL9185 (HDMI PHY, mux and EDID) and then to MT8226. So the passthrough might be possible in MT8226 firmware.
Try to find if such hack exists for TV platform where is used only MT8226 (AMBER, PEARL ??) and not STi710x. Even if such hack exists, MT8226 firmware
for A7/8/9 is substantially different.