Up to now, I have only an (stupid?) idea:
Example:
frame what one sees
1 P
11 PE
21 PET
31 PETE
etc.
for each part an object (P, PE, PET, PETE ,…) and two materials: one for visible objects one for invisible object (alpha = 0).
Use a python driver to change the material at given frame numbers.
(Though I am not able to realize this yet, because I miss an easy example of creating, manipulating a driver and connect it with a suitable script )
with blender-2.55
text-object
with material and a moving texture with alpha-channel.
The texture lets the material change its alpha-value
and make the letters visible.
Its only possible with mono-spaced fonts or you have to adjust
the moving of the texture (by object, normaly an empty) by hand.
If its a monospaced text-font like the capital letters one below,
it is possible to setup a “simple” step-function for the moving
empty-object …