My thought is to maybe cut it off flush or more likely just proud of the grip. Then I could wrap the stub to the new blank for a little extra support. Depending on the blank I could open up the handle. I’ve a 53’ trailer my new team uses for automation installs with a mill in it and a few other goodies. Could probably get a local machine shop owner to use a horizontal mill to bore it out too. Guess the question then is how to recreate the original. I’m guessing it’ll have to be a lighter profile rod.
Suppose I should start looking for another 8’6” Imperial 5wt. It may even be an 8’.
Talking out of my ass here. I know F all about rod building and my machine shop experience is almost 30 years old, limited, and way out of date.
Is there a lathe in the trailer? I think that would work better than a mill. Chuck it up carefully, center it well with a dial indicator if the lathe chuck isn't self centering. Put a really thin drill bit in the, um, thingy at the other end. Advance it slow and careful.
Step.up the drill size a fraction and repeat, slow and careful. Keep stepping up a fraction at a time until you get to the diameter you want.
Probably a wood lathe would be a better choice than a metal lathe. Never used one so I don't know anything about them but I can foresee problems using machine tools designed for metal on a non metal rod handle.