Not the same thing but there is this project that does digital rgb to hdmi using a pi https://github.com/hoglet67/RGBtoHDMI https://github.com/hoglet67/RGBtoHDM

Not the same thing but there is this project that does digital rgb to hdmi using a pi https://github.com/hoglet67/RGBtoHDMI https://github.com/hoglet67/RGBtoHDMI I believe they use a custom firmware on the pi and a CPLD, but you could probably eliminate that doing hdmi to hdmi.