If you have maxed out mortgage deductibility (taking out 100k for non housing purposes) can you count the intertest on additional takeouts which are put into a 529 plan as investment expense?

I don't know of any IRS guidance on this question, but I think it is doubtful you can claim interest as investment expense. I believe the IRS would view it as personal interest, because the "gift" you make to a 529 plan is personal in nature. Your tax advisor may have a different opinion.

Joe