It’s only OS-specific insofar as the only OS that can currently communicate with the EC is Linux. I’m working to change that!
That’s an excellent question. I can offer up my experience here, as I think I’m the only Framework Laptop owner who’s running custom EC firmware[1].
Yes, it is open source. In my free time, I’m working on building support for QMK-like layout customization here (which has been extracted into its own repository for easier future ingestion into other ECs using the same codebase.)
I’ve been using it as a daily driver for about a month now, and everything that’s implemented today works the way it’s supposed to. Here’s some source code glamor shots:
[1]: Having also written a tool to assist in reflashing your EC safely: Announcement: ECTool.efi - talk to the EC from the UEFI shell (and reflash it!)