the documentation doesn't contain an instruction on how to provide public mirror for sources archives for overlay. see also https://bugs.gentoo.org/790506
Also, the section https://devmanual.gentoo.org/general-concepts/mirrors/index.html#restricting-automatic-mirroring should be made more clear. 1) If ebuild is located in overlay, should it contain RESTRIT="mirror" ? 2) If not, then how to avoid searching on gentoo mirrors? 3) how to force searching on overlay mirror instead of original location without prepending another second SRC_URI line?
see also https://github.com/gentoo/guru/issues/41