If you're about to write a filter anyway, and I assume that you are going to tell people that their input has been "censored". Why not add a function that looks for PRE and TT tags, this is probably someone who doesn't know about the code tag. We could then point out that using code tags has many other benefits, allows people to wrap the enty, download the code directly etc.

This way we could help educate new people who obviously dont realise that the <code> functionality is there.

Nuance