Overlays for routers (especially OnHub)?

I’ve started looking into what it might take to get OpenWRT running on my old Tp-link Onhub routers.  One of the first steps I was hoping to take was to get a dev build of ChromiumOS running on the router (rootfs verification disabled, more debug logging, etc.).

Are these routers closed source in a way that other ChromeOS devices aren’t?  Is there any chance of these overlays becoming open source at any point?  What leads to an overlay being closed or open source?

I did find a thread from way back in 2018 with some similar questions, but no resolution.

Thanks for the help!