Why doesn't /usr/libexec/java_home recognize JDK 1.8?
You need to also copy the Contents/MacOS directory and Info.plist from the 1.7 SDK and adapt them (by changing the version strings in it).

Then you can:

$ export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home
$ java -version
openjdk version "1.8.0-ea"
$ /usr/libexec/java_home -v 1.8*
/Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home


Note that this is a hack and I didn't try anything else than start java... I was just building on your tip of creating the package.