Double dashed bar lines

[Evan Driscoll]

Disclaimer: I realize I'm on an older version of Lilypond. If that makes
things substantially more difficult it's not out of the question I could
upgrade, but I'd rather not at this time.

I'm trying the following:
    \version "2.20.0"
    \relative c'' {
      c4
      c
      \bar "!"
      c
      c
    }

and this works, it gives me a dashed barline between the second and third
notes.

However, I can't figure out how to get a double dashed barline. \bar "!!",
"!|", and "|!", as well as `\bar "!" \bar "!"` are my four guesses, but
none work -- I get no barline with any of the first three, and just one
with the doubled-up `\bar`s.

Is this possible? What's the right markup for this?

Evan