That (PDEPEND) dependency is just for javac and javadoc, as far as I can see in the ebuild. But when jamvm is installed, it's probably to serve as pure JRE. I suggest keywording the gjdoc/eclipse-ecj dependencies.
I agree, jamvm makes more sense to have as a pure JRE. If a pseudo JDK based on jamvm should ever be needed, the same approach as for gcj-jdk could be used. Ie. add as jamvm-jdk and symlink what is needed. Fixed in -r2.