I know I could do it with custom luma/wipe file for each letter - these are really hard to do (I've got scanner handwritten letters and only a mouse to draw them with) and would require a lot of time to do them just right. Is there any other way in Kdenlive?

You can write your text on a drawing program (e.g. Gimp) and capture the video of the whole process.
Does that answer your question ?

good call - the problem being that I didn't write it and I don't own a tablet to write it with