ossia staff without SystemStartBar / Bracket / Brace

[musicus]

> I'm not top posting.

Is there an easier way to get an ossia Staff without SystemStart than 
following code? 
I'm thinking of something like SystemStartBar #'allow-span-bar = ##f 

\version "2.19.17"

<<
  \new Staff = "ossia" \with {
        \remove "Time_signature_engraver"
        \hide Clef
        \hide KeySignature
        \magnifyStaff #2/3
  }{a a a a b b b}
  \new StaffGroup {
    <<
    \set StaffGroup.systemStartDelimiterHierarchy =
        #'(SystemStartBar (SystemStartBracket (SystemStartBrace a b)))
       \new Staff { a a a a b b b}
       \new Staff {a a a a a a a }
  >>
}
>>

\layout {
  \context {
    \Score
    \omit SystemStartBar
    \override SystemStartBracket.style = #'bar-line
    \override SystemStartBracket.padding = #-0.1
    \override SystemStartBracket.thickness = #1.6
  }
}