Created attachment 277727 [details, diff] base.eclass.patch base_src_install() should support custom destination directory, so that functions, which call this function internally, be able to perform installation into a different directory than ${D}. An ebuild might want to firstly install files into e.g. ${T}/${subdir} and next specially merge them into ${D}. I'm attaching the patch.
base.eclass has to be removed. Don't see the point to apply any patches.