For example, I've got a root object exposed in a SOA service, say Invoice (Invoice has line items as children).

Sometimes, I need to retrieve its detail line items. I'm thinking to make it lazy loading, because it's a traffic overhead to transfer line items every time Invoice is required. But in SOA fashion, it seems unlikely. Because all it can expose are Invoice POCOs, with contain no logic. Thus I cannot attach my lazy loading logic to Invoice to instruct it to load lines items when needed.