Thanks Jay - that's what I thought you might say. I've settled on an approach that actually seems to work pretty well. I have a flag (True or False as the case may be) and if the flag is changed then I invoke a plugin restart. If the flag is True, subscribe -- otherwise don't subscribe.
Mine is a pretty fringe case for the Multitool plugin and won't seem out of place given its intended purpose.
+1 for unsubscribe.