Space before staff lines in ossia

[Jonathan Wilkes]

Hello,
     Here's a picky little question: what's the best way to get the staff 
lines of the ossia measure to extend, say, half a staff space before the 
first note (*without* changing any of the current spacing between notes)? 
Ideally I'd like to be able to set this in a \layout block.

-Jonathan

\version "2.12.2"

\score {
        \new Staff = "main" {
          \relative c' {
                c(\mf\< d ees f )\ff |
                \key d \major
                fis(\pp\<
                <<
                {
                  g a b)\mf |
                }
                \new Staff \with { \remove Time_signature_engraver
                                   \remove Key_engraver
                                   \remove Clef_engraver
                                   alignAboveContext = "main" }
                {
                  \key d \major b c d |
                }
                >>
          }
        }

}