EOP license for on premises users in a full hybrid

I think you don't need to. According to article https://docs.microsoft.com/en-us/exchange/hybrid-deployment/move-mailboxes#what-do-you-need-to-know-before-you-begin

The Microsoft 365 or Office 365 Exchange license must be assigned only after the migration is complete. You then have 30 days to assign the license.

I think the license is for the Exchange Online mailboxes only once it is migrated. For the mail flow going in-and-out of your on-premise Exchange Server, HCW (Hybrid Configuration Wizard) automatically creates an inbound and outbound connector in Exchange Online Admin Center for your Exchange Server. 

Also this might help: 

For general overview of Exchange Hybrid, you can refer to https://docs.microsoft.com/en-us/exchange/exchange-hybrid.

For prerequisites of Exchange Hybrid, you can refer to https://docs.microsoft.com/en-us/exchange/hybrid-deployment-prerequisites.

For mail flow, you can look into https://docs.microsoft.com/en-us/exchange/transport-routing.