Sweetbay Magnolia is not a very common plant so you haven't gotten responses yet. I know there are several at the Frelinghuysen arboretum in Morristown and I am pretty sure they are several at the NJ Botanical Garden in Ringwood. You may contact them. I don't know if they let their plants host caterpillars though. They might spray everything. Also there are a few butterfly societies around if you were to google NJ Butterfly or Butterflies, you will get a couple of hits. This site mentions a book published by Rutgers in the late 90's.
Otherwise, you would have to experiment yourself. If you offer it and it is a known host, you have a chance. There are no guarantees though. I never got Monarch cats on my butterfly weed.

Here is a link that might be useful: Checklist of NJ Butterflies