The project is called “Tactility” and its website is https://tactility.one/
You can run ELF binary apps directly from an SD card without restarting or flashing the ROM. There’s an SDK for building these apps, but I haven’t made an official release yet.
I wrote a blog post with some background information: https://bytewelder.com/posts/2025/01/06/tactility-one-year-later.html
Source code and project files: https://github.com/ByteWelder/Tactility
My friend runs a similar project in Rust, focusing on running wasm-compiled programs