Ah, I see. I have to turn on global source preview to select source point. Well, it kind of works, but is far from perfect:
* I can't just Alt-Click on canvas. On Win7 I have to first click on global source, then source point, which is a lot of cursor moving. On Mac, I have to double-click on global source and then set source point.
* When global source preview is on, you see global source only, no your local fixes already done, so I loose what I was going to fix, especially given I have to move cursor far from fix point (see item #1)
* There is no "current layer and global source" variant
So I still think it would be really cool to have some way to specify exact source layer or at least "current and below in folder".