The jaxb reference implementation provides two tools, xjc and schemagen. xjc is installed by our dev-java/jaxb-tools package, but schemagen is not. The main class for this tool is com.sun.tools.jxc.SchemaGenerator which already gets compiled by the current 2.1.2 ebuild, so you'd only have to add another launcher.
Cross references: bug 188007 about bumping jaxb to 2.1.4 bug 188009 about about making the jaxb ant tasks available
Created attachment 127172 [details, diff] add java-pkg_dolauncher for schemagen to jaxb-tools ebuild
Any update since 2007? Do we want this ebuild to make it into the tree? You know the drill: comment or close.
jaxb-tools has been removed.