If you used joints on the teeth and the tongue, you could control there movement by coding them to the sliders, but I myself think that that would probably not even give good enough results.

If it was me, what I would do would be start over and either :

Do all the texturing first, then combine the whole head (using polygons), then do the manipulation for the blend shapes,

or, (an I think this is the best solution) start over and model the blend shapes around the teeth - but have the teeth parented to some joints so that when you do an open mouth, you know where the teeth will be and can fit the face and lips to corrispond.

That is how I would do it anyway . . .