This morning without putting a whole lot of effort into this, I was able to winnow it down to 27.32 bits per word without any other trickery like 5bit packing. You need at most 1 bit per word to identify the real words, so that's 27.32+1 bits without the bitpacking. Doing 6 bits per letter drops that by 25%.
Now having put too much effort in, I'm around 21.6 + 1 bits per word, (17 bit packed) just by using SSP.
After throwing more words at the Google Wall, it finally allowed that what I'm thinking of is the Shortest Superstring Problem.