OCF says,

All other files within the OCF Abstract Container MAY be in any location descendant from the Root Directory, provided they are not within the META-INF directory.

but also says

This directory contains the files specified in META-INF Reserved Files. Files other than the ones listed in that section MAY be included in the META-INF directory;

I just made an EPUB with a content document in META-INF. EPUBCheck did not complain, and the file worked as expected in several reading systems.

Perhaps we should delete ", provided they are not within the META-INF directory."?