The most common practice is to treat a 529 or ESA distribution as parental support (or, at least support, not provided by the student), as the parent is the owner of the plan. Making distributions to the owner not the beneficiary, or the school, would reinforce this position.
The treatment of expenses paid with distributions from Sec. 529 plans and Coverdell ESAs in the support test is uncertain because of the dual nature of these college savings vehicles and, as you already noted, a lack of IRS guidance.
References:
https://www.savingforcollege.com/articles/the-impact-of-529-plans-on-claiming-a-dependent
http://www.aicpa.org/publications/taxadviser/2010/august/pages/nichols_aug-2010.aspx
Other discussion in this forum.
https://ttlc.intuit.com/questions/3402602-can-i-claim-money-from-a-529-fund-as-my-supporting-my-22-y...
