However you can produce secure brain wallets with for instance a mechanism involving a large number of different hashing functions.
I doubt it. If enough people start doing that, password brute forcers will do it too, the same way they use combinations of words, phrases, common transformations, etc. All you are doing with these sorts of combinations are adding a relatively small number of entropy bits for the various ways that hash functions can be combined.
Key stretching does work but that relies on the hash function being slow, which is somewhat fragile long term.
Right, but this is just one example. The point is to have some semantic to apply to your password, that you never disclose. For me a mini computer program could do the trick but for somebody else it could be something else.
But this is not practical or advisable for the masses, I agree.
EDIT: in the example scheme of having N repetitions of hashing functions, it also has the additional personal advantage that I can choose roughly how much time I'll need to get my secret key. (Assuming I don't put my hand on ASICS for all hashing functions involved). So for an important brain wallet I can make it roughly 24h on a typical computer, and be sure I wouldn't spend it drunk.
