I'm shooting a flick in 3/4, but I want to add fake 16:9 bars afterwards, in After Effects. Now I'm wondering, does the 16:9 frame extend all the way to the edge of the 720 x 576 frame (PAL) or should it end at the point where the "action safe" borders are - after all, thats where the tv screen ends.

When adding fake bars, extend them all the way to the edge - even if you don't see all the way to the edge.

Ok. Not that I doubt you or anything, but why is that? Does the image have to extend all the way to the edge of the frame?

And does it really matter? Say I invent some kind of specific ratio that I happen to like, will I be unable to show my movie on TV?

Ridicilous questions, I know...

Even though a TV won't show it, if someone looks at it on a TV with a smaller overscan area than yours, they may see it. Also if you want to render for the web, you would see it.