Well Spalding plays acoustic bass - not much slapping going on there...
Prior to this, the real bass threat out there was MeShell Ndegeocello, who sings, raps, and plays funk bass- However MeShell is real scary looking while Esperenza Spalding has this cute Minnie Ripperton vibe (must be the huge 'fro).
I found this other female bassist on YouTube on a DIY video. Very very good singing and bass playing (she plays chords along with the bass line), doing Marcus Miller's version of Denice Williams' "Free". (On Marcus Miller's album the singer was Corrine Bailey Rae.)
see