That reminds me of the sage advice Bruce Schneier wrote in Secret and Lies, "Anyone who creates his or her own cryptographic primitive is either a genius or a fool. Given the genius/fool ratio for our species, the odds aren't very good."
He's saying to build a custom function out of well established cryptographic primitives.
I don't think that's a good idea either. There's still a ton that can go wrong that won't be apparent at all to someone who knows how to analyze entropy flows through the internals of hash functions. There's a reason why we have a PBKDF2 that's favored over the original PBKDF.
