Right now, StructureDefinition identifies the input and output structures essentially at the schema level - what elements exist that can be moved around or looked at. However, StructureMaps will often be based on a more constrained version of that schema. The schema might be CDA, but the StructureMap requires a constrained template on C-CDA, or the schema might be QuestionnaireResponse, but the StructureMap requires that the response be for a specific Questionnaire. It should be possible to point (likely by URI) to the StructureDefinition, Questionnaire or other artifact that defines what the input must be valid against or the output is expected to be valid against. This won't have quite the same rigor as the formal pre-conditions and tests embedded in the FluentPath and other assertions within the StructureDefinition, but it's a lot more useful for searching for a relevant StructureMap that will produce what you need or consume what you have.