longa of two perfect breves under shiftDurations and Completion_heads_engraver

[Graham King]

Is there a way to make the following MWE generate a dotted breve tied to a 
dotted breve, rather than breve~semibreve~breve~semibreve ?
 
I would prefer not to abandon the use of shiftDurations or state "c'1. ~ 1." if 
these things are avoidable, for weak reasons to do with my strategies for 
transcribing mensural notation.

\version "2.19.82"

\shiftDurations #-1 #0 {
  \time 3/1
  c'\breve.
}

\layout {
  \context {
    \Voice
    \remove "Note_heads_engraver"
    \consists "Completion_heads_engraver"
  }
}

Grateful, as ever, for your kind suggestions...

-- Graham