Mobius Strip Tech has posted an unofficial PC Engine Duo to HD Retrovision adapter. Just solder a MiniDIN9 to the PCB, then wire a pigtal of a DIN8 adapter to the board. There’s even a 3D printed housing for it!
Please keep in mind that this is an unofficial converter and HD Retrovision will not support any consoles they don’t directly make cables for. Also, you’ll need to make sure your RGB mod compensates for the differences in the Genesis 2’s output signal, otherwise it won’t work correctly.
Overall, I’d consider this an “advanced user” mod, not because of the skill required to assemble, but because of the knowledge and understanding of that mod that’s needed to execute this properly. All official HD Retrovision pigtail adapters (except the Genesis 1 / SMS one) have carefully engineered circuitry inside them to ensure a perfect signal is passed through to the Genesis cable, so any 3rd party adapters must be carefully tested before use.
PCB Link: https://oshpark.com/shared_projects/amKj4zix
3D Printed Housing: https://www.thingiverse.com/thing:3048576