Fill of each letter of the word, like progress bar.

Edit:
Actually, EZGUI has a UIProgressBar control you could hack a little to get the effect much more easily! Give that a look if using EZGUI. NGUI probably has a similar control. idk why I didn't think of that first; I'm even doing something like that in my game >.>;; /Edit

As far as I am aware, there is no built in functionality for this. However, there might be an addon on the asset store.

If not, you will probably be able to fake the effect by using ui tookit like EZGUI or NGUI, and exporting loads of frames for each size of letter, setting it up an animation of some kind. For EZGUI you could use PackedSprite, then telling the sprite which frame to show. Note that this way will be very heavy on texture memory (probably around 20-30x the next method).

Doing it properly, however, will be pretty difficult/time consuming. You'll need build and resize a mesh in code (ui library might do this for you), then manage uvs based on a font file (probably BMFont) you'll have to parse to get the raw uv for the letter, then tween according to a percentage and letter you'll set via a script.

Another way might be to directly edit the textures, copying what you need onto a buffer that you display, which will require creating textures in code. Since you won't have to worry about resizing the mesh, this might be easier to implement. Not sure which would be faster though ^^;;