Well, fixed it! Big thanks to Nic at Fallline Cycles in Peebles, we bounced a few ideas around and he had a great idea about using a connectamajig nut.
If anyone has this problem in the future (which I doubt anyone’s dumb enough to do this).
Here’s what I did…
I cable-tied a 35tpi hacksaw blade to some dowel, then glued a Stanley blade to the end. I cut the rubber boot off the internal routing, which exposed the barb’s thread, then removed the blade.
I then sawed a cut in the internal routing hose.
I got the nut from a connectamajig, sawed it in half, then chamfered some dowel of around 7mm, then drifted it in, and glued it in place.
I then took a 6.5mm wood bit, and drilled out the middle of the dowel.
I screwed the tool onto the exposed reverb barb thread and pulled. The internal routing snapped clean off and the hose came out.
Win.