\version "2.12.2"

fb = \figuremode {
  % Question 5: If I use figuremode in a Staff context with extenders enabled, how can I use markup in the figures?
  \bassFigureExtendersOn
  <\markup{"myfig" \number 6}>
}

music = \relative c' {
  c1_"figure markup in Staff" |
}

\new Score {
  \new Staff <<
    \new Voice = "FiguredBass" { \fb }
    \new Voice = "Music" { \clef "bass" \music }
  >>
}