Re-aligning instrument names

[David Sumbler]

I have spent a long time trying to find the answer to this, but without
success.

I have a piece with strings divided often into several parts; sometimes
these may not be even divisions, for instance where a four-part section
has instruments 3 and 4 playing the same music.  I therefore need to
show which individual instruments are to play each line.

\version "2.19.81"

\new StaffGroup \with {
  instrumentName = "Violins I" }
<<
  \new Staff \with {
  instrumentName = "1."}
  { c' }
  \new Staff \with {
  instrumentName = "2."}
  { c' }
>>

In above minimal example, how do I get the Staff.instrumentName-s to be
right-aligned to the system bracket, rather than centre-aligned as the
StaffGroup name is?

David