I've not yet made a cored shaft; but it has occurred to me in thinking about it that one approach i might take would be as follows: Make the initial dowel somewhat oversize of the intended wrap area diameter. Cut a tenon the size desired for the core on the forearm end, extending back into the wrap enough to cover the joint. Make the fore arm solid, a little long for the centers & to clear cutters on the over-travel. Keep everything concentric. If the wood in the forearm is difficult or expensive, it can have a core smaller than the final size of the tenon installed, to create the length needed between centers on the dividing head.set up.
Build the forearm between centers & when done turn it between centers with a minimal amount of extra diameter for later clean up. Really minimal, but some needed. Then chuck in a collet or any method you choose to keep it concentric both ends in the lathe/steady rest, etc. and bore it to fit the tenon. I would not make it too tight a fit. In WEST I trust.
Then let it kick around the shop for a period of time to see if it stays straight.
The problem with a full length dowel is that it ends up behaving about like a full splice. The wood can move when it is turned. You just never know.
When ready to work the full length blank with the slightly oversive forearm on it, put a drill bushing in a collet in the lathe, and a 4 jaw on the nose over that. Set the blank up in a 4 jaw at each end and dial it in so the points run true at both ends of the forearm. You can cheat a little at the wrap end to keep the butt cap end from being too offset, if necessary. The eye can't catch a few .001's off center there the way it can at the sharp end of points due to the taper ratio. Be certain the chucks are not bending the shaft, and be certain the points are running true.
When it is all dialed in, Stick a long straight drill with a (shop made) half-side center angle tip through the drill bushing in the headstock & poke a center in the blank. Cut a true land/tenon on the other (butt) end leaving it large as possible dia at this time. Take the revolving 4 jaw off the TS & put a center in. Put the new stub tenon end in the headstock with the recently-poked forearm center on the TS center. Dial the round land/tenon on the butt sleeve end back in, in the 4J, and poke a center it it, too through the headstock bushing. The lathe should be running, as well as the portable drill motor to drive the D-bit through the bushing.
Now, if the shaft stays straight while being turned, it can be worked between non-influencing centers with a driver dog, reversing or not as useful.
Cut the wrap area and see if the fore-arm moved. Hopefully not, but if so you need to decide whether to do the whole 4j + collet + bigger/new center poke. Finish the wrap diameter, skim the forearm to size, and cut the tenon for the butt sleeve and butt cap.
It should make a very solid cue. Since i have only made full splices and merry's the set ups are familiar. But it's a lot more work than what i perceive an A joint to entail. If the cue does not have points to keep aligned, it is actually a simple proposition.
smt