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The server would have to perform some equivalent of the -d and -f test on every request, and pore over your htaccess to make sure the request isn't coming from someone who will end up being blocked (core comes after all mods including rewrite). You're looking at a significant detour into a php script for every single request, because a server-level redirect on its own would happen before the request ever reaches your individual site.
We get constant errors in GWT like the ones described above