biblatex ignores own lbx file

I’m trying to build an own biblatex style named musuos for that I created the two style files musuos.bbx and musuos.cbx which look like the following:

\ProvidesFile{authoryear-musuos.bbx}[2011/05/06 v1.0 Musikwissenschaften Uni Osnabrueck]
\RequireBibliographyStyle{authortitle}
...
\endinput


and

\ProvidesFile{authoryear-musuos.cbx}[2011/05/06 v1.0 Musikwissenschaften Uni Osnabrueck]
\RequireCitationStyle{verbose-ibid}
\ExecuteBibliographyOptions{
autocite=footnote,
labelyear=true,
dashed=false,
urldate=long,
}
...
\endinput


If I load them with \usepackage[style=musuos]{biblatex} everything is fine. But now I need to adjust a language string. For that I built the file german-musuos.lbx and saved it in the same directory as the style files.

\ProvidesFile{german-musuos.lbx}[2011/09/10 v1.0  biblatex localization file]

\InheritBibliographyExtras{german}

\NewBibliographyString{bibcitedas}

\DefineBibliographyStrings{%
bibcitedas = {{zitiert als}{zit. als}},
urlseen = {am},
}

\endinput


But it seems that the lbx file isn’t loaded. What do I have to do to make biblatex use it?

-

Skimming the files of biblatex-dw, it seems you have to include the following code line in authoryear-musuos.bbx:
\DeclareLanguageMapping{german}{german-musuos}

BTW, the authoryear prefix seems to be improper for a style that combines features of authortitle and verbose.