I just finished writing patches that should enable you to use the
following DEPEND:

C_DEP="
	dev-java/sun-jaf
	!java5? ( dev-libs/cyrus-sasl )"
DEPEND="
	java5? ( >=virtual/jdk-1.5 )
	!java5? ( >=virtual/jdk-1.4 )
	dev-java/ant-core
	${C_DEP}
	source? ( app-arch/zip )"
RDEPEND="
	java5? ( >=virtual/jre-1.5 )
	!java5? ( >=virtual/jre-1.4 )
	${C_DEP}"

http://dev.gentoo.org/~betelgeuse/patches/java-util-2.eclass-use.patch
http://dev.gentoo.org/~betelgeuse/patches/versionmanager-use-flags.patch

Of course we could do the parsing using our own code instead of using
portage directly. IMHO it would be the best to have some kind of API
ebuilds could use against the package manager to do these kinds of
things. It's a bit hackish so I don't know yet if I will commit these.

Regards,
Petteri