I have used solid plugs of fiberglass in the layup when I knew where I was going to be putting a threaded attachment or if I was going to have a lot of compressive load like a through bolt. I used solid round fiberglass rod from these guys:
http://www.mgs4u.com/fiberglass-tube-rod.htm , cut it to the same length as my core material thickness, bored a hole with a hole saw to the right diameter (I used 1" but they have up to 1.5") and epoxied it into the core. Then did the whole layup. Seems to work well - FG plugs can even be tapped/threaded (but not as strong as threading into good metal).