As Derek Wade pointed out, you can make an application like GarageBand X (which is awkward with third-party plugins like Amplitube due to the sandbox) DOES NOT run in the sandbox by editing the binary itself using a HEX editor such as HexFiend. Search for:
<key>com.apple.security.app-sandbox</key>
Right after this bit you will see the true tag, which, as I suggested, switched to "fals" (without extra bytes), and now GarageBand will happily interact with third-party VST plugins. Exclamations of glee.
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