I very much like the rapid hoisting and unhoisting with FI & FO in the repertoire of Birch commands.
Might it be possible to add some kind of visual cue to remind the user that focus is narrowed ?
(Analogous to the top-left corner triangle in FoldingText)
And could one add a constraint on focusing in to the point where no text shows at all ?
( Just in case one returned to what looked like an empty buffer and lost track of the invisible text – it can be a little startling if one hits FI one time too many and suddenly nothing is visible at all : - )