No, they cant be keyed, i dont know what on earth possessed you to think you could. Keyframes imply its somesort of mathematical value that can be interpolated. And im pretty sure no one has yet taken the concept of interpolating words up yet. You can offcourse keyframe allmost everything else in maya because its numeric in nature.
infact theres little idea of pushing tetuual data trough the dg, altough would certainly be possible to implement. But on the border of pointless, since it imutable on its way so one mightaswell push it the whole way by himself. Angain such mutators could be made, but taht too is kindoff pointless.
Anyway it is certainly possible to change them outside of the dg context with setAttr -type "string" like construct, now you can put this intoa expression node but as said it works outside of dg and thus has certain update problems associated with it.
hmm what would be the inetrpolated value between cat and dog with a catmul-rom iterpolation? Elephant?