.mmi (mixed media image) support

https://www.gametechwiki.com/w/index.php/Pioneer_LaserActive

NEC PC Engine add-on and Sega Genesis add-on for this Pioneer LaserActive laserdisc home media player let one play games on LaserDiscs, a combined digital-analog disc format. Digital in-game graphics is rendered by console hardware of these add-ons on top of analog video. Here's the description:

https://techdocs.exodusemulator.com/Console/PioneerLaserActive/Laserdisc.html

The technical side of archiving process has finally been dealt with and now is considered polished enough:

https://techdocs.exodusemulator.com/Console/PioneerLaserActive/Archiving.html

The "Mixed media image" was developed for dumping these discs, this format combines digital content and captured raw uncompressed analog video signal into a single .mmi file (hence why huge sizes up to ~40GB per disc side), several games have already been dumped.

Currently, the author is adding support for Mega LD games in emulator Ares also on GitHub, but NEC LD-ROM² is 100% identical, the same dumping techniques apply, and its support planned as well.
edit: And GPGX may add it, see https://github.com/ekeeke/Genesis-Plus-GX/issues/603.

I ask to add support for this format to BizHawk for both Sega Mega Drive and PC Engine emulation.

Start with the linked LaserActive technical and archiving documentation, then trace BizHawk's Sega Mega Drive and PC Engine emulator entry points. The issue names no BizHawk files or tests; done would mean .mmi support works for both requested emulation targets with appropriate coverage.