So, I'm uncomfortable making a long-term promise to support krb5-config in a multi-arch environment. Today, the multi-arch change is easy to work with. However, it seems entirely reasonable that the recommended cppflags and cflags might differ between architectures, and I don't know that I want to sign up for the work involved in supporting that long-term.
This sounds like a wishlist bug on the face of it. But it sounds like you view it as more important. so, why don't you take this opportunity to try and sell me on how cool multi-arch is for krb5 dev packages and why it provides Debian with neat enough capabilities that I want to commit to supporting it. Regardless of how convincing you are, I can apply this patch for now, but I may label multi-arch support for the -dev packages as experimental and subject to future removal if it proves to require more effort than the package maintainers want to spend.