stringnumbers on upperstaff?

[Jonatan Liljedahl]

Is there any way I can have stringnumbers appear above the upper staff
in a PianoStaff? 

For example:

\version "2.7.28"
\new PianoStaff <<
 \context Staff = up {
  \skip 1
 }
 \context Staff = dn {
 < c e\3 > 2
 \change Staff = up
 < d f\4 > 2
 }
>>

Now the stringnumber (3) will be shown above the bottom staff but I'd
like it to be above the top staff... Is this possible? Could I maybe
move the StringNumber engraver outside each Staff and put it in
PianoStaff and somehow adjust the Y-postition??

/Jonatan    -=( http://kymatica.com )=-