Hello Paul,
[...]If I'm writing music in F, then I suggest that I be able to use bF as a pitch instead of bf. The F would indicate that all subsequent bs would be flattened until one is encountered with a different accidental or until the end of the current music _expression_. It should have the same scope as \stemUp or similar. [...]
I don't know "how much Frescobaldi knows" of the lilypond code the user is editing. If it has a logical representation of the source code it could be Frescobaldi (and not lilypond) to handle this feature and offering to autocorrect, according the key signature indicated in the source code, the note you write while you write it.
You are in F, you write b and it propose bes.
Maybe with different language (never used english for lilypond note input) this would be more difficult.....
Ciao, g.