I never said anything about Onyx, I don’t own one but have considered them. They look nice and open.
I do own a couple of Kobo devices though and just wanted to say it’s not running Android of any kind but it’s still relatively open. Especially compared to phones, tablets and Kindle. The firmware/OS point you’re trying to make is irrelevant there and I think you know it :)
There is NickelMenu and you can telnet into it. You can also install other OS like KOReader easily, it doesn’t have a locked bootloader or anything like that. So imho that’s pretty accessible and open.
No it’s not running Android.
Using Calibre you could probably glue that together. I wouldn’t want Android on an ereader personally.
It’s not open source but it is easily rooted and you can install custom add-ons or even replace the os.
I believe that’s their usual release process.