> these programs operate as you describe
Okay, then they *do* use (essentially) the same method as Lilypond,
not some "visually-oriented" method which "follows the key signature"...
Not so. In Sibelius, you put the key signature, e.g. F sharp major, then type
the plain letter names, e.g. f g a b c d e f which plays back as the scale
of F sharp major. The Lilypond method seems a bit odd to start with,
but "es" and "is" are easily typed. What's the point of quibbling over it.
As Graham says, the coders got there first.
So is there *any* example of an application which tries to "follow
the key signature" for someone?
Yes - Sibelius.
Not only do I know of no such program, I can't even imagine how it
could be done (technically).