Bfa-Bmi key signature?

[Graham King]

In transcribing a piece of renaissance music, I am trying to refine the following key signature:

\version 2.19.21
\relative c' {
  \set Staff.keyAlterations = #`(((-1 . 6) . ,NATURAL)
                                 (( 0 . 6) . ,FLAT))
  R1 b2 bes b' bes
}

Ideally, the flat in the key signature should have a ledger line, and should be vertically aligned with the natural sign.  (There is a corresponding keysig in bass clef which also needs this alignment)

I have looked at:
1) The re-writing of the key signature engraver in Scheme: https://lists.gnu.org/archive/html/lilypond-user/2015-08/msg00429.html


2) The addition of ledger lines to a key signature: https://lists.gnu.org/archive/html/lilypond-user/2014-11/msg00454.html


3) The Notation Reference http://lilypond.org/doc/v2.19/Documentation/notation/displaying-pitches#key-signature and the sections of the Internals Reference that this refers to;

and I confess that I'm floundering a bit.  Well, more than a bit.  

This specific issue is quite common in early music, where the composer uses the hard hexachord.

Grateful for any help/guidance !

-- Graham