As for binary searching for first letters in a sorted dictionary, I've done that in my submission to the word ladder problem which was a problem in perl quiz of the week expert edition week 22.

However, I don't quite see how that would help in this word selection problem. Unless you somehow preprocess the dictionary, you cannot select a random word in constant time even if you know the offsets of the first letters, as just choosing a random offset favors long words. So, IMO, you have to read the whole or most of the dictionary file for this problem unless it's preprocessed in some way. Random access to the file may still slightly help after reading it completely, but not very much unless there are very long words in the "dictionary".