The remarkable 2, especially if you downgrade the OS to the older 2.x versions, is very hackable. It runs full Linux, I followed a blog post about setting up FDE on it using go and cryptfs [1]. You can make GUI applications, people have even run XFCE, and people have fed handwriting input to another API so routing it internally shouldn't be much harder.

So yes, theoretically you can turn the reMarkable 2 into a emacs lisp machine, although it would take a considerable amount of time.

I haven't read up on the reMarkable Paper Pro, but I think the dev ecosystem for that is very much alive too.

[1] https://blog.redteam-pentesting.de/2021/remarkable-encryptio...