I ended up restricting the "requires excluded" hack to the specifically named modules after all. It turned out to only affect the two modules I had found: Darwin.C.excluded and Tcl.Private. I tested this back to OS X 10.8 and iOS 7.
This preserves backwards compatibility for two hacks in the Darwin system module map files: 1. The use of 'requires excluded' to make headers non-modular, which should really be mapped to 'textual' now that we have this feature. 2. Silently removes a bogus cplusplus requirement from IOKit.avc. Once we start diagnosing missing requirements and headers on auto-imports these would have broken compatibility with existing Darwin SDKs.