Thank you all for your input. This is one of those things I would have given up on in fear of making it worse before having joined the Looth Group. The quality of information shared here is amazing.
As it turned out, I couldn’t finish the job unless I fixed the walnut plug issue, as the truss rod was overly tight and the stop point for the rod to engage the other direction to add relief was the insert. It was in there snug. I ended up cranking the truss rod to push it free. I then drilled through a dowel about the same size of the plug and tapered the end to fit against the plug’s contour. Then, I epoxied in a 1/4″ metal dowel through the the center to help with indexing when pushing (or potentially tapping with a hammer) for the glue back.
I lightly sanded the plug with 220, tightened the truss rod as far back into the channel so to allow clearance for the plug. I waxed off the truss rod nut and lined the cavity with fish glue before reinserting. After some touch up with a mix of Zinsser and StewMac Vintage Amber, I used Glu Boost Fill & Finish to seal it off. If I could undo that last step, I would liked to have sanded a bit of the hard edges of the broken finish to better feather in the fill. The customer was beyond happy with the final results so I’m onto the next one and will file this one in the memory banks for the next time.
You guys rock. Thanks again.