You can poll using JMX via ruby or java or perl, there are JMX client libs for most languages.
But if you wish to use the JMS API, then the statistics plugin, which is real time, could be for you. It responds on the replyTo destination with the latest stats.
I think you have identified a nice enhancement to the statistics plugins. An option that wold cause it to periodically output statistic information to a destination with an expiry, such that it could be picked up at any time. Alleviating the need to pool for it to happen. This is something that could be considered in a future release.
And, yes, you can mix and match broker configurations in a network, but not the stores used for destinations within a single broker.
I managed to pull apart the JMX examples and get them working.
MQ stats don't seem particularly reliable though, but at least I can get at them now.