I used to carry my wife on my previous B4 all the time -- daily commute. She eventually got pretty comfortable with it and became a very good passenger. The trick is to get her to snuggle up real close and HOLD ON. Then go for smoooooth operation, not necessarily tame operation. Smooth shifting, gentle roll-ons isntead of whacking it, easing into the brakes etc. If she's holding on real good, then she should have no trouble. A jerky shift is way more troublesome than the pull of hitting the powerband.
I also used to have her actually lean on me a bit -- it would get heavy in hard braking, but gave me better feel for what she was experiencing.
Of course this was all on my B4 and the extra weight often forced my further into the powerband.
A big liter bike tourer is that way for ease of highway riding and the ability to add a passenger without significantly degrading performance, I guess.
that's my .02