In most case, you don't need this, though. Can you prove that your construction is secure? How many hashing algorithms are there? Remember that big O thing. Just increasing the workfactor for scrypt will be much better than using a combination of 5 or so hash functions.
"Unique algorithm ... in order to make development of highly optimized password brute-force technologies a "per-site" exercise for attackers" for KDF/password hashing is similar in principle to secret algorithm for ciphers: consider that the attacker already has an optimized brute-force technology for your combination of primitives (for ciphers: consider that the attacker knows your algorithm); replace this unique algorithm with the work factor/secret key to have fewer moving parts.
I think a clearer way to express the author's position is to consider the case where a feasible attack is discovered on (say) SHA1. If you consider the probability of an attack on SHA1 and blowfish to be of independent probability, then the author's scheme allows you to "hedge your bets", as in theory your algorithm is as strong as its strongest component, even if the strengths of its components change in the future due to new discoveries.
This line of reasoning assumes A) that the probabilities of attacks on the algorithms are independent, and B) that the algorithms in use do not substantially reduce their input entropy, both of which are potential attacks.
It does _NOT_, however, assume that the scheme be kept secret.
If you really want a "custom algorithm", the better advice would be to use a known algorithm with a secret key.
For example,
https://wiki.mozilla.org/WebAppSec/Secure_Coding_Guidelines#...
Why? See http://en.wikipedia.org/wiki/Kerckhoffs_principle
In most case, you don't need this, though. Can you prove that your construction is secure? How many hashing algorithms are there? Remember that big O thing. Just increasing the workfactor for scrypt will be much better than using a combination of 5 or so hash functions.