I think I've dug as deep as I can with this on my own but it really doesn't seem it's possibile to control the feedback for an operator with the LFO or Envelopes. It's only the modulation amount I can control (and I guess that's the same effect as controlling the level of the operator, right?). I can however control the feedback with for example the Auto LFO midi insert from Cubase on CC 86, CC103 etc) It works fine but then that can't be saved with the patch.
It would be awesome to control the feedback with the individual Envelopes. Or, since I'm new to FM-synthesis, maybe my approach is wrong? Can you get the same results another way (not based on external CCs)?
...or could it be possible to have flexible destinations (including feedback) with Envelope or LFO as source in a future firmware-upgrade for routing modulation?