Very possible, very annoying for everyone involved in creating it.
The script would have to detect any letter combos, like ch and sh, but the decal packs would be so infuriating to make because you'd have to make 5 different decals for the five major poses;
:I closed
:0 Semi
:O open
:D wide, teeth visible.
:F teeth, "F".
Aaaaaand any incompatible faces without mouths or whatnot can't work.