Is there anything I can protect my Ivy with during tent fumigation? I’m thinking maybe a coating like Nims oil?
The Plants in question are 30 years old, cover over 1,000 sq. ft, with some have a 4" diameter trunk. They are entrenched to the Structure.
I don't have first hand experience with a situation exactly like yours, so I'm 'educated' guessing, but here is what I would do:
Talk to the pest control company that is doing the fumigation. In California they are required to have a licensed applicator supervisor who knows the products being used and environmental hazards. He/she may have had experience with plant protection like this. If not, the state Structural Pest Control Board may be able to direct you to someone who does. https://www.pestboard.ca.gov/
Also, the UC Extension may be of help.
Find out exactly what they will be using and if it will kill the stems of the ivy. It may just burn the leaves and the plants may refoliate in spring.
I wouldn't count on neem oil for a protective coating, but consider this product: https://www.amazon.com/gp/product/B01LZ2O26A/ref=s9_dcacsd_dcoop_bw_cr_x__a_w
Protection from termite fumigation chemicals is not the normal use for this material. Contact the manufacture for pros and cons.
Here's some general information, although the article does not address your situation where you cannot move or protect the ivy attached to the home.