You can ask for any of the sides that are a part of other meals instead- I think the onion rings are extra so they would not be a subsitute.- you can pay for them however as an extra side.
in general, we have found that if you want to substitute something similar that comes with another entree they will allow it...but something that is listed as a side dish or an appetizer with a separate price will not be subbed for the veg that comes with an entree/combo platter.
So for example at 50s Prime Time, some other entrees come with either green beans or sweet peas. I would bet they would allow you to sub those types of green veggie for the collard greens...but not the onion rings, which are listed as an appetizer.
We've done this many times at other Disney restaurants...and sometimes they will even just allow double potatoes (or double rice, or double noodles if that's the starch with your entree) if you don't care for the other veggie offered with other entrees.
They won't give you onion rings because you are supposed to pay extra for those. But you can ask for a different veggie. You can also ask for all white meat chicken - love that. I think I will get an order from the bar on Star Wars Weekend and box up a breast to carry to the parade area.
They will substitute another veggie, but not onion rings. I will say that I didn't think I would like them, and they were one of the highlights of the meal! (and I ate every bite!!! )