Of course, the obvious other way to have the figured bass on top is to simply type it first in your score. In this case, you wouldn't want to add it to the same staff context as you have done it. Maybe this is what you're looking for? Here the vertical placement of the figured bass is independent of the music:
%%
\score {
<<
\continuoFiguren_IX
\new Staff = "Continuo" {
\new Voice {
\clef "bass" \global_IX \compressMMRests \continuo_IX \bar"|."
}
}
>>
}
%%
