Well, guess what, I found it !
Finally found a working YouTube app that works on the Libre Touch, not only does it work, it actually does it really smoothly, and the videos can play at full screen! What I did was just to Google up "Youtube app for Android 1.5", one of those search results led to an APK download somewhere on a blog.
If anyone is interested and don't want to take the hassle of hunting it down, let me know.