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.

\version "2.20.0"

#(set-global-staff-size 16)

\defineBarLine "|.-dropout" #'("|." #f "|" )

\score {
  \new StaffGroup <<
    \new StaffGroup <<
      \new Staff { R1*32 \bar "|." }
      \new Staff { R1*32 \bar "|." }
      \new Staff { R1*10 \bar "|.-dropout" }
      \new Staff { R1*10 \bar "|.-dropout" }
    >>
    \new Staff { \clef "alto" R1*18 \bar "|.-dropout" }
    \new Staff { \clef "bass" R1*2 \bar "|.-dropout" }
    \new Devnull { s1*6 \break s1*6\break s1*6\break s1*7\break s1*7 }
  >>

  \layout {
    \context {
      \Score
      \remove "Timing_translator"
      \remove "Default_bar_line_engraver"
    }
    \context {
      \Staff  
      \consists "Timing_translator"
      \consists "Default_bar_line_engraver"
    }
  }
}