Forgot to consider that; along with the fact that the 5 random characters can be any printable ASCII characters, not just alphanumeric.

So this is somewhere upwards of 10^50 phrases to hash (in a naive implementation), right?

Well, are you also including the fact that there is nothing forbidding repeated words?

While this has a marginal effect on the total number of possible hashes, repeated words are still an important consideration: If you can guess even one word that is likely to appear in their 1000-word dictionary, you can precompute the hashes for all possible variants of the corresponding phrase (that word repeated twelve times) prior to the contest, giving you a jump on other contestants.

I'd guess that we'll either see the rules revised to prevent this or see a dictionary filled with non-dictionary words.