There's no mechanism to read an individual system property that isn't part of the persistent configuration. I thought java.lang.management.RuntimeMXBean exposed that, in which case /core-service=platform-mbean/type=runtime would as well, but I was wrong.
Can you file a Feature Request JIRA for this? I'm not certain where we would expose this; most likely under /core-service=platform-mbean/type=runtime.
As for "include-runtime", that refers to whether or not runtime-only attributes of a resource are shown. In the case of system-property=java.version, the resource doesn't even exist.