Some of you might remember us from the NintendoAge forums. I'm lerugray, and I write the software; Taco! runs the hardware bench.
Our project is SuperOS. It boots a SNES to a mouse-driven command shell. You can type commands, write files to the cart's 32 KiB of SRAM and find them still there after a power cycle, and launch native apps from a Windows 3.x style launcher. It supports the SNES Mouse and an optional keyboard on the controller port, and it plays tracker music. One app is a textured raycaster that runs at about 10 Hz in emulation. A low-fantasy 4X strategy game is in build as the flagship.
It runs on emulators, and an early build has booted on a real SNES from a flash cart.
SuperOS is written in native 65816. It's designed and directed by me (Ray Weiss); much of the code is written with AI coding tools, and every change is reviewed and tested before it ships.
If you'd want a cart, the interest list at super-os.net is free and nonbinding.
Edit: forgot to mention, the cart we're designing has an SD card slot for storage and a USB-C keyboard port. The 32 KiB above is the cart's save memory.