Hello - is it possible to make items exclusive to certain customers?

No

What about workflow or script?

You can restrict items by using a saved search on the sales order form. So you would need a sales order form for items that don’t have any restrictions and then another sales order form for that includes in the unrestricted items and the items for the specific customer.

There is a feature called Personalized Catalog Views for SuiteCommerce that does this. My hope (dream?) is that NetSuite will extend this PCV functionality outside of SC as well. We could absolutely use it, and I'm sure many other customers would as well.