I think it makes the most sense to designate the person currently maintaining the package/PKGBUILD as the maintainer irrespective of that person's status in the community or the destination of the package/PKGBUILD. It immediately indicates to anyone looking at the PKGBUILD whom they should contact about updating it.
I think previous maintainers should be listed as contributors along with anyone who's contributed signficant changes to the package. Telling people that they can't claim to be a "maintainer" of a package because they're not a dev or TU comes across the wrong way too. Just because the binary isn't hosted in the AUR doesn't mean that the work of maintaining a package (updating, responding to comments, etc) is any different than if the binary were uploaded. Just my 2¢.
