Er, that footnote says that those are *precluded*, not permitted (though of course as a footnote it is not normative).
I'm not sure how you can implement these requirements without either ABI changes or an efficiency reduction (on platforms where sub-maximum-sized writes are inefficient). We might have to eat the efficiency loss: we surely cannot fix this by aligning every member of every structure to cacheline boundaries or something like that, that's ridiculous.
(But I am a neophyte in this area. Someone, anyone, who knows more than me, please chip in!)