I throw buns on top of the dogs in a steam pan all the time. It just takes a few seconds when the pan is really steaming when the buns are naked. I grill my Nathans and store them in the steam pan.
 
I even wrap a few buns in my foil wrappers and put them in the steam pan if I see a few people starting over to my cart. That works good too. They can stay a bit longer in the steamer pan when wrapped in the foil sheets that I use from Sam’s Club.
 
In the wrapper they are not taking in all the moisture as they do when naked. When naked you have to watch them closer because they will get soggy and even stick to the metal pan. I try to put them on top of the dogs when naked to avoid them from touching the metal and sticking to the pan.