EAP runtimes defined in stacks don't show up as download runtimes. They don't have a download url but have a human readable url. As in JBDS 6 (in central), they should appear as a valid choice and display the human url so the user can manually go and install the runtime.
I also noticed that if the download url is missing the download runtime wizard refuses to finish (url not valid error). It should close without any error.