I would find the careful use of a heat gun or better yet, low pressure steam much more practical and accurate than repeatedly trying to dip a section of 9' mast in a 2' bath of actively boiling water. Though it could be done outdoors, it also could be awkward and dangerous.
If it's ABS, the furling ring may have a lower melting point (about 220º f) than the mast (if 275º as Tom suggests) or even the epoxy. So it may be the plastic that starts to break the bond first. This all makes for an interesting experiment.
It's easy enough to mix up another batch of your epoxy and test it first to see how much heat is actually needed, or if it helps at all. You could glue some ABS parts together if you want. Use a simple oven thermometer to practice achieving the right heat prior to starting.
You MIGHT be able to heat the entire ring enough to knock it loose with a soft mallet, but I think the real trick is going be to first score the ring into sections with cutting tools like blades and dremel (mutli tools also work great for this) and then use steam or dry heat on each section. Don't cut all the way through to the mast!
Clamping the edge of the ring in a vise would allow you to gently pry pieces off, as the bond softens. (I don't like the idea of hitting a carbon mast with anything harder than my head).
Have someone assist you, always work with gloves and safety glasses, stay hydrated, blah, blah, blah,..
And document the whole thing. We'll go make some popcorn.