This way of working does save a lot of time - and it's much more fun animating using the picture rather than the f-curve editor. This animation took about 8 hours to do.
Horror: I think I've set up all the shots wrongly. There's an "Output format"-preference in XSI that I didn't notice when I set it all up. The worst thing is that the frame rate is wrong (NTSC instead of PAL) and the time line becomes screwed when fixing this. I'll see how I'll overcome this.