I would like to alter the COPY command

What I have is data with two different characters for "start quote"
and "end quote". In my case it's '[' and ']', but it could be
anything from "smart quotes", to parentheses, to brackets, braces, ^/$
in regexps, etc. I think this isn't too unreasonable a feature to
have to make copy more functional when importing data that is
difficult to transform properly beforehand (in my case is about half a
terabyte of log files, which takes hours and hours, just to cat, let
alone reparse and dump into COPY).

Now, in my case I can just say "cat file | tr '[]' '""' | psql -f
import.sql", but then I lose the ability for psql to do anything smart
like using mmap (I'm making assumptions that it does anything smart
like that, but even if it doesn't now, it could some day).

So, I'm a passable c/c++ programmer, when I have to be, so
theoretically I can do the work myself, but I have never touched
postgres before, so I don't know where to begin. Any ideas how to add
this?