I know this is a very old post but i thought i'd reply just in case. :)
I have a pretty old version of the Betty Crocker Cookbook which has a pineapple variation on the original candied yam recipe. The original recipe is as follows:
INGREDIENTS: 2 pounds sweet potatoes or yams (about 6 medium), 1/2 cup packed brown sugar, 3 tablespoons margarine or butter, 3 tablespoons water, and 1/2 teaspoon salt.
DIRECTIONS: Prepare and cook sweet potatoes as directed. (Wash first, then heat enough salted water to cover potatoes to boiling. Then add potatoes. Cover and heat to boiling. Cook until tender, 30 to 35 minutes; drain. Slip off skins.) Cut crosswise into 1/2-inch slices. Mix brown sugar, margarine, water and salt in 8-inch skillet. Cook over medium heat, stirring constantly, until smooth and bubbly. Add sweet potato slices; stir gently until glazed and heated through.
PINEAPPLE VARIATION: Omit water and mix in 1 can (8 1/4 ounces) crushed pineapple (with syrup).
As far as the oven temperature and time is concerned, this doesn't say to put it into the oven at all, and it also doesn't mention marshmallows. I would imagine you could cover it and bake it for like 20 minutes at 350, and then throw some marshmallows on top and bake it for five to ten minutes more or until the marshmallows are golden. But...maybe you should taste test it first before you bake it. You might like it better than the way your grandma did it. :)
Hope this helps.