Hello,
Certainly: if your document background image was tagged in InDesign with an XMPie Graphic Content Object, the uProduce SOAP API "SetCustomization" method from the "JobTicket" class will let you specify the path of the customer's uploaded image, thus letting the composition engine use it as a value for this Content Object, instead of using the original background - that I suppose was set in uPlan.
I assume that the v1/jobs/submit method of the REST API would roughly do the same, even if I do not claim having too much personal experience with it.