I've done this when someones expression changes mid sentence a couple of times in my hack. But for full blown animation it wouldnt work, seeing as the fast left to rigth scrollign motion of the facepic would distract from the intended effect.
The only way i can see of making a flowign animation in that manner is via assembly. But i dont see how it could be worth it as the work would be fairly considerable in assembly and the fact that you would be runnign otu of facepics fast.
Sayign that, it might be nice to have an assembly driven 3 frame loop facepic for an important scene.
There are better more efficient ways to animate though, and i'd rather stick to them.