I think I do understand, this is exactly what we do. (MEGA_MACRO == HWY_NAMESPACE)

Then we have a table of function pointers to &AVX2::foo, &AVX3::foo etc. As long as the module exports one single thing, which either calls into or exports this table, I do not see how it is incompatible with building your project using modules enabled?

(The way we compile the code twice is to re-include our source file, taking care that only the SIMD parts are actually seen by the compiler, and stuff like the module exports would only be compiled once.)