I've cooked potatoes on the grill before... but I've par boiled them 3/4th done or so first. That way when whatever I'm cooking is about ready, I toss on the potatoes and crust them over in just a few minutes... with the corn, tomatoes, etc. .. whatever else I've tossed on. But not wrapped in foil.
Red bliss and Yukon Gold work well for this... but anyway, I've never wrapped a potato airtight in foil and "baked" it on a grill before... I'd be inclined to take a fork and poke holes in the foil so some smoke or whatever flavor would permeate the potato... hummm... Interesting... I haven't met a vegetable or fruit that isn't great on the grill
But this thread is about onions.. sorry to get it off topic... BTW... our Walla Walla onions should be ready in a few weeks... can't remember the season now. We have Vidalias in the store now... I doubt they send the Walla Walla onions all over the US like the Vidalias.. but they are equally as sweet and mild, IMHO...