What if you add your own personal coding to the obvious phrase?
Lets say, replace all the letters A with B.
For example "cbptbining finbncial conservbtism mbyonnbise" instead of "captaining financial conservatism mayonnaise"
Will it more difficult to get the key?
Maybe a bit, but not really.
An attacker with the skills and resources to create and scan a precomputed list of brainwallets based on the most common words and phrases will likely also start scanning the most common permutations eventually.
So it's safer in the sense that the coins will probably only be snatched after a couple of days instead of after a couple of seconds.
Granted, given a long enough passphrase or a complex enough "cipher" your coins should be reasonably secure. However it's hard to guess at which point this is the case, which is why one should resort to more reliable methods. It's probably not at 4-word phrases with single-letter-replacements though.