I don't think our eclasses are allowed to assume a specific package manager in order to work. Afais the provided feature of the eclass could work for arbitrary locations, it doesn't really have to be "/etc/portage/savedconfig". Maybe it makes more sense to stuff it somewhere into "/var" so it is common across all PMs?
/var is for state data, not config files *** This bug has been marked as a duplicate of bug 202411 ***