Can I use chopped white onion instead of pearl onion?
I have seen recipes that call for chopped white onions instead of pearl onions, which leads me to believe it is feasible. 

The texture might be different; when cooked, pearl onions tend to have a different sort of bite to them, much like a grape, revealing a burst of flavor after a slight resistance, whereas chopped fullsize onions tend to be more one-note and soft. I don't know for sure what pickling does to the texture, but if it is beneficial to use whole onions for a similar effect, you'll lose that. I suspect the flavor penetration would be greater, however, due to the cutting.