I have to do something like that! I am a little worried about breakages though.
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OpenBSD malloc and hardened_malloc treat it as if it's a 16 byte allocation but within a dedicated PROT_NONE memory region. It avoids violating the requirement to return a unique pointer. There are definitely programs depending on it.
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It's also not possible to return NULL in practice despite the standard allowing it. That's why it makes sense to return a pointer to a PROT_NONE memory region. It does need to be unique to avoid breaking compatibility though. It can even be problematic for it not to be aligned.