I suppose that windows specific things stay in Platform/* files as now, but some general items common for all targets and architecture should be moved into Compiler/* files. I will prepare new files for Windows first but what will be moved into Compiler/* it will be shared with other platforms.
Anyway one question, CMake implement OW with WCL386 tool, it is only driver that it is inefficient it is middle layer between wmake and compilers. Better should be implement compilers directly from wmake, because wmake has direct interface to compilers (DLL) that use one instance in memory and call compiler directly by internal interface. But it create incompatibility between old (WCL386) and new (WCC/WCC386/WPP/WPP386) implementation (different options, some are same). But if OW stuff will be in Compiler/* files there is no problem switch between old and new implementation.
Probably best will be use old implementation to introduce coross-compilation features and next discuse this change.