Although I don't have an alternative to offer you, lag bolts into epoxy in a balsa cored deck just sounds like a bad idea to me. I agree with RB, balsa has no real strength, even with epoxy. I have always through-bolted things using 1/4" aluminum backing plates. Cleats, stoppers, blocks, etc. I think that would be a good candidate for your epoxy in the balsa, but I also agree that it is also a possible leak source just like any other hole through the deck.