What's a good nondairy substitute for parmesan/grana padano as a salad-topper?
The main things that Parmesan cheese can add to a salad are fattiness and salt. The first thing that comes to mind for me is bacon. You could probably also make a yummy main dish salad (or hearty first course) by adding duck or a cured meat such as salami. 

If you're not interested in adding meat, how about olives? Those would serve similar purposes. Other options might be marinated artichoke hearts or mushrooms. Just make sure you choose ones that aren't too tart, as they might add to the effect of the lemon vinaigrette.