The (originally for Dreamcast) can be adapted to NAOMI. This taps the digital RGB signals before IC501 and converts them directly to HDMI using an FPGA.
Here is a detailed report based on the available technical data: Component Type: Integrated Circuit (IC). demul mpr- 21931. ic501
IC501 is a component on the MPR-21931 board. Unfortunately, specific information on IC501 is scarce, but it is believed to be a part of the board's graphics or sound processing circuitry. The (originally for Dreamcast) can be adapted to NAOMI