Q. May I make a non-qualified withdrawal from a 529 plan, without paying the 10% penalty, for non-taxable employer tuition reimbursement?
A. Yes. Having received employer assistance, in the amount of the distribution, qualifies as a penalty exception. Reference: page 53, IRS Publication 970. https://www.irs.gov/pub/irs-pdf/p970.pdf
Q. What about any taxable bit?
A. The amount would still be taxable. While there are exceptions to the 10% penalty, there are no exceptions to the tax on a non-qualified 529 distribution (rollovers are not taxable). Reference: page 53, IRS Publication 970.