Most postings to a WBS are for cost postings and there is not a direct relationship to a customer. For example, when a Purchase Order posts costs to a WBS there is no customer involved.
The relationship between customer and WBS is through the account assignment on a SD document.
This sounds like a custom reporting requirement. If you have a WBS hierarchy you won't have a direct relationship from a WBS to a SD document (ie, Sales Order, Sales Contract, etc.) for the lower level WBS. In your report you will need to trace from the WBS up the WBS hierarchy to the Billing WBS that is account assigned on a SD document. From there you can get the Sold-to Party Customer.