Cubase doesn’t currently offer any similar way to work with loop points within a audio sample like found in ableton and bigwig, so in Cubase the process is going to be more labour intensive. One way to do it is the way you suggest above (“copy the pattern until…”). At the moment I’m experimenting with regions, where I open a audio sample in the audio editor, in the Hitpoint menu I create regions. Then I open up the Pool window and drag regions to the track in the order I desire. It’s even more time consuming and it doesn’t compare to ableton, but it can give you quite interesting result.
As for programming polyrhythms in MIDI, I posted this some time ago (which none took notice of ): Steinberg Forums
): Steinberg Forums