Surely the Orders service would support a single entrypoint - "index for customer X", or something - which would return a list of links into the resources that

Surely the Orders service would support a single entrypoint - "index for customer X", or something - which would return a list of links into the resources that it owns? You're not forcing the Customers service to know anything more than that single per-customer entrypoint. Exactly the same pattern applies for any other domain. All the Customers service needs to know is that there is a link to another domain; the structure is entirely down to the service for the other domain to manage.