Using sed to replace all stdout from within .bashrc

I want to conduct a sed operation on all output in my bash shell. Basically I want to put the sed command in .bashrc so it "listens" for certain keywords to do something with them.

For instance I am looking to put something like this in .bashrc:

sed ''/critical/s//$(printf "CRITICAL")/g''

So that anytime the word "critical" pops up, it is changed to "CRITICAL". This might be when I cat a file, am using vi, or am telnetted into another system. What do I need to do to get this to work?

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