it is absolutly illegal to use anything to warm the food except a wood fired pit. You may hold in a cooler or kambro to keep it hot until prep time, but nothing using propane or electricity and that includes microwaves or gas fired ovens.
For me, during a contest, ribs, pork and brisket are usually coming out of a cooler after resting for a bit and are still to hot to handle, and chicken is coming right off the pit.
If needed, you may return it to your pit to set glazes and warm stuff up a bit before going into the boxes, but thats it.. NO ELECTRIC OR GAS.
honestly, if you use a cooler or kambro to hold the meats, there is NO reason to heat it up. They are very hot when the time comes. Also, keep in mind, the hotter the product when it goes into the box, the more they will wilt the greens. When u close the box, it becomes a little steamroom.