Most hotels that have at least a continental breakfast provide a microwave, toaster and/or toaster oven for general use, not just by those who partake of food provided on premises, included in the room rate or purchased, but all guests. I can understand that the placement of these amenities in food courts and lounges on the Disney properties can cause some confusion about the acceptability of their usage with food brought from outside the grounds, but I assure you that not only is it kosher, but it's even expected behavior by the resort.
Toast away, comrade. And do so without fear of repercussion.