Another approach would be to simply state the instructions either in a blob of text preceding the music page or perhaps somewhere between the title and the music:

This though would require translation should the work be published for e.g. Klingons or Lojbanists who may be familiar with classical music scoring conventions but not English.
% LilyPond engraving system - http://www.lilypond.org/
\version "2.18.0"
\header {
title = "Vuvuzela Concerto in D"
}
\markup {
\vspace #2
\italic {
"Play entire piece softly ("
\dynamic mp
") except for measure 7." }
}
bbbb = {
\tempo 4=96
bes1~bes~bes~bes~bes~bes~bes\ppp
}
\score {
\new Staff << \bbbb >>
\layout { }
}
(Redundant information may also help future translators link language to the musical notation of the time, and will otherwise help reinforce the composer's intentions—however bizarre they may seem.)