How do you hold polenta hot for service?
The only way I've held creamy polenta was in a splayed edged sauce pan covered with a cartouche. I would make it looser than necessary as it does loose some moisture through out the shift. It was held either towards the end of the range or on a side of the grill that was turned off to keep warm. It's easy to work when it's warm, added a splash of water when necessary and put the pot back on the flame when the order is fired. Very good results.
Like Pete says, steam table, covered. Add a little stock or water periodically and give it a stir.
Also, make fresh daily. Don't use last night's polenta for soft polenta. Instead, at the end of the night pour it out, onto a sheet tray and chill it overnight. Then either pan fry and use in a dish or cut into cubes and deep fry and use a croutons for a soup or a salad.