I made some progress.
https://github.com/Hyphen-ated/korea...af398a4c580a09
Adding configurable actions to page through the file browser was easy.
I also unlocked the ability to configure the top-left and top-right corners, which had been intentionally disabled because the Home and Plus buttons are there. But I want to be able to have top corners that are like, 1/4 the width and height of the screen, so it makes total sense to have these usable even when there's also a tiny button there.
I'm not sure how to make the user-configured tap areas take priority over the item_table entries representing files. Right now my big corner tap areas work great, as long as there's fewer than a full page of files so my tap doesn't get eaten. I haven't found any explicit Z-ordering in the code. Maybe I need to do something in some object's onMenuSelect?