I'm working on this requirement from my EU client. We are on Magento 1.9.1
We have set up VAT validation and auto-group change based for valid and invalid VAT IDs and its working fine.
Now the twist is, we want to validate VAT for customers who are not logged in and move them to a customer group say "Not-Logged-in-Valid-VAT"
Is it possible to assign guest to a customer group since guest is not actually a full customer entity ? Is it possible to replicate the concept of Not-Logged-in customer group ?