You could taper a big piece of delrin and use it on the outside of the forearm while you press the core in.Depending on the size of a tapered core, I would be afraid of a little to much pressure installing the dowel and ending up with a split fore arm. Truthfully, I'm quite happy with a .750 straight dowel. As long as the centers in the core are true, the prong wood needs to be no thicker at it's thinnest part than what is needed so that it's not translucent.

I'm with you, if your core and 3/4" hole are cut right, (including centers) there's no danger of the forearm being too thin.