Make every n'th barline different?

[Graham King]

Is there an efficient way to change the style of every, say, third barline?
Here's an example of what I'm trying to achieve, but I'd like to do it without having to insert \bar "|" manually (and of course with real music rather than \repeat unfold):

\version "2.19.2"

\relative c' {
  \set Timing.defaultBarType = ";"
  \repeat unfold 4 {
    \repeat unfold 3 { c c c c }
    \bar "|"
  }

bar.pdf
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