The eclass does: einstall INSTALL_ROOT="${D}" which pretty much proves that it supports DESTDIR in form of INSTALL_ROOT. Therefore, einstall should not be used here.
As far as I can tell nothing should be even using this eclass now. Is there any reason to keep it around? If we have to hang onto it for the sake of uninstalling old versions of mythtv then I'd suggest ignoring this or just making the install phase a no-op.
@mgorny: Know the policy on removing old eclasses? Honestly I can't remember the last time that eclass was used. Probably 4 years ago?
Thanks for the report. Fixed in http://gitweb.gentoo.org/repo/gentoo.git/commit/?id=e30816724e4daaf09e6404c5cad5701826c2a1bc