There's no reason you couldn't sign a DVD ripping app; all that code signing will do is ensure that you are who you say you are. Since you don't have to distribute apps through the Mac App Store, you could sign up for a dev account, build your custom app, and distribute it however you want.
For tools like that, this could actually be a big benefit - it lends a sense of credibility. You wouldn't be able to distribute modified (read: backdoored) binaries, and if you did people would know who you were (at least, to some extent, and Apple could revoke your credentials so no one else would inadvertently run your software).
As for pirated software: I haven't seen a lot of actual 'cracks' lately; mostly it seems to be 'put in this serial plus set these hosts entries so it can't phone home' sort of thing. For app-modifying cracks, you'd have to disable this, but I'm not sure how common that is lately.
But one of the things Apple can now do is void you keys and make all the apps on people computer unusable at least as I understand it. So for legally dubious application this wouldn't be such a good idea.
