I would say this is radically not locked down. You can ssh it!
If they tried to accommodate custom “mods” it could crash their updates. I don’t want them spending dev cycles on such an accommodation.
Besides my desktop computer, this is the best technology purchase I’ve ever made. I continue to use it for practical things.
Ultimately the functionality didn't interfere with the built in reader software or OS because it was entirely orthogonal the extra functionality was more icons on the launch screen. It didn't need the developers to create much in the way of extension points save for the expectations that the launch screen wasn't hard coded but rather it would let you select from whatever apps were installed. Something pretty common in basically every OS.
If your work doesn't have some degree of natural separation between A and B I don't know how you can possibly have made it work in the first place.