Maybe I'm being incredibly naive, but it seems like this would be trivial. Can you just start with the output hash and then essentially run the algorithms backwards? Obviously the resulting "input" would be random-ish garbage, but it seems like if all you care about is the output, you can pretty much just "pick" any data for the last step that produces the output. Then do likewise for the step prior, and so on.
As a comment above stated, part of the "input" is the initialized values:
> Initialize hash value h0 to h7: first 32 bits of the fractional parts of the square roots of the first 8 primes 2..19).
My guess is h0 to h7 change throughout the algorithm. If you perform each step in "reverse" as you suggest, "picking" any input at each step that produces the required output for that step, then you may not arrive to the correct initial state with the square roots of the first 8 primes.
