All-purpose bass tablature

[address@hidden]

Hi,
I am getting close to what I want, and yet the last bit eludes me. I am 
regularly transcribing bass parts from performances played on 5 or 
6-string basses. Invariably my intended audience is a 4-string player 
and so needs either a modified part or a pencil.
My idea is to accommodate all basses on a 4-string TabStaff, by using 
ledger lines to represent the low-B  or high-C string notes, while 
allowing 4-string players to see the difference. It seems this idea is 
not original. Lilypond-user Laura Conrad was asking the same thing for 
lute in 2003.
It does appear that in 'Internals' there is a mention of 
ledger-line-spanner in TabStaff but I'm not good at internals. For 
instance, I am confused that changing the line positions on the tab 
staff requires Staff.StaffSymbol and not TabStaff, so I guess one must 
be a sub-thing of the other.

The following builds a tablature for 6-string bass to enable the outer 
string fret numbers.
The staff is then redrawn with just the middle 4 strings.
The low-B string is chorded with its substitute note an octave higher, 
and the fret numbers appear correctly.

The chord construct wouldn't work for high notes as I couldn't obscure 
one notehead
of the same note within a chord, so I opted for the temporary 'multiple 
voice' construct.
The low note and the high note needs a tab ledger.

% BTW this also works with the stable release
% \version "2.18.2"
\version "2.19.83"

num = {
    \omit StringNumber
}
stm = {
    \omit StringNumber
    \omit Stem
}
pattern = \relative c,,  { < c c'> f bes ees aes << \stm des\1 \\ \num 
des\2 >>  }
<<
    \new Staff {
        \clef "bass_8"
        \pattern
        %\pattern
    }

    \new TabStaff \with {
        stringTunings = \stringTuning <b,,, e,, a,, d, g, c>
    }
    {
        \override Staff.Clef.font-size = #-2
        \override StaffSymbol.line-count = #4
        \override Staff.StaffSymbol.line-positions = #'(-3 -1 1 3 )
        \pattern
        %\override TabNoteHead.no-ledgers = ##f
        %\once \override Staff.StaffSymbol.ledger-extra = #2.0
        %\pattern
    }
>>

It occurs to me that I might duplicate the ledgers from the top staff 
and severely reposition them, but it would be a last resort.

Any feedback welcome.

Paul