The only thing that prefixing the length makes difficult is using the same prefix multiple times: you basically have to make up your mind about the type and length before mounting the shattered attack. Also, the prefix means you have to do your own shattered attack and can't use the PDFs that google provided as proof of their project's success. Price tag for that seems to be 11k.
I'm curious about this too considering GitHub uses it for GitHub pages, I would assume they have a vested interest in maintaining Jekyll even if they had to fork it to improve upon it.