This was discussed in MQTT-276 with notes from the F2F meetings and has been tracked in MQTT-256.
I'm opening a separate, specific issue per Ken's comments - https://issues.oasis-open.org/browse/MQTT-256?focusedCommentId=62192page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-62192:
"All of these would be defined in separate JIRAs, but what we should do in this JIRA is to define the mechanism used to pass these values."