Is it passable to stretch certain body parts towards something?
Did anybody ever played left 4 dead?
There is a zombie called "smoker".
I noticed they simply used spirits to perform the long tongue effect.

Any ideas?

Should work with cloth function as one-way mode.
Make a certain body part as cloth and attach it to your model.

What happens if you use scale_x/y/z over time?

No, but how about adding segments to the extending body part dynamically so that it grows towards its target? I can't comment on efficiency or performance but I think it could work. For example, if you wanted an extending arm you could split it into two parts: the arm and the hand. Then you could add arm segments over time and then add the hand once you have extended it enough towards its target.

in the video , i think they use a bone and effects.
the bone move to target position and the effect started from the old position to target position.