Diveney maybe?
Diveney Cues might do it, give them a call. He has made me a 4" oak extension that I play with on my cue all the time and a 2 piece 8" extension in case I need to reach way out there. They have Pats quick connect pin and joint and it screws in through the hole in the bumper on the butt. A very slick set up.
I'm thinking he could maybe core the extension for you to lighten it up to the weight you want. Pat is a talented guy and honest and does good work. Give some thought to a two piece 8" extension with the first half cored and the second half there for the longer shot.
When I get my other cue butt and extension back from Pat I am sending the 4" oak extension back to him for a 3/8 ebony and 3/8 copper ring on each end and his beautiful clear coat finish.
It will look good on the Custom Ebony Hustler with some small but very well done ivory inlets he did for me and the dark Diveney LD Kiel Wood shaft.
Joey in Cali has done a couple small jobs for me and you could PM him and see if he could do it. He is a stand up guy and reliable.
Let me know how it goes please and good luck!