Update: I'm also having trouble seeing the utility of this proposed checker. Suppose I have two arbitrary web applications A and B, and both were kind enough to supply xsd schemas. Can I determine if they can interact meaniningfully? Not really. It's a matter of semantics, not just matching schemas.
I hope the OP answers this, because its been interesting so far. But for the calendaring application mentioned, why not create a simple schema and require conformance? It would need to be done per client with xslt (the glue language of xml!) or whatever. But without explicit agreement you are just guessing that elements from different schemas with similar names describe the same real-world objects. Sounds dangerous.