Bar line definition with annotation

[Timothy Lanfear]

\defineBarLine with annotation changes the span bar type when the bar is 
in the middle of a system, but not at the end. A modification of the 
example from 
https://lilypond.org/doc/v2.20/Documentation/notation/bars#bar-lines, 
does not give a dashed span bar at the end of the system.

\defineBarLine "||-dashedSpan" #'("||" "" "!!")

\new StaffGroup <<
  \new Staff \relative c'' {
    c1 \bar "||"
    c1 \bar "||-dashedSpan"
    c1 \bar "||-dashedSpan"
  }
  \new Staff \relative c'' {
    c1
    c1
    c1
  }
> >

Any suggestions to work around? The question arises in the context of 
Haydn's Farewell symphony where instruments drop out with a double bar 
appearing in one part with normal span bars in the rest of the system. I 
attach a skeleton of the last page of 
http://imslp.eu/files/imglnks/euimg/8/82/IMSLP31493-PMLP51750-Haydn-_Sinfonia_Nr45_(HCR_Landon).pdf, 
showing unwanted span double bars in the third system.

--
Timothy Lanfear, Bristol, UK.

farewell.ly
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