Adding a temporal 3rd staff (for only 2 bars) in a Piano Staff

[fauban]

Hello Lilypond Community!
I am typesetting a piano score. 
As most keyboard pieces, it begins with 2 staves, but measures 14 and 15
would need a 3rd staff over the other two. (I am using the new PianoStaff
command)

I have tried to make something like an ossia staff, but it's not the same,
as its bar lines are not connected to the ones of the two main staves.

Here is an example of what I want to do (Debussy: Préludes - Book II, No.1;
taken from the IMSLP. Check the 1st page):
http://imslp.info/files/imglnks/usimg/d/dd/IMSLP00510-Debussy_-_Preludes__Book_2.pdf

Can anybody help me?

Thanks!!
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