In theory at the Federal level just about anyone can open up a 529.
But in practice under state plan rules there is almost a universal requirement that the plan owner have a SSN or TIN etc.
However note that the donor need not be the owner and that many plans accept such funds.
my LAY thinking:
So if you as parent have a SSN or TIN you or somebody close with same could open up a 529 and grandparents contribute to same.
My thinking is that such foreign donor would not be subject to US gift tax issues absent a treaty on point and ssaid person could make essentially large contributions right up to the overall plan limits which may be in the 200K range.
Note that if the child or the parent is the owner it can have serious financial aid prejudice issues. Other trusted family members might be better nominal owners.
The nonowner donor lacks control over changes in beneficiary or actual distributions--this may or may not work in your family. The actual owner could do some strange things with the account! You sort out your own family!
A point I have not seen addressed is if there can be any required qualifications of a successor owner. The transfer of ownership is a nontax event. The original concept was to transfer ownership by will--and there is nothing to say you cannot pick a foreign person to inherit under ones will. This has been greatly modified by most states to permit the transfer of ownership by plan designation and for any number of reasons or sometimes no reason at all. Ergo I have no idea if there is any bar to setting up the plan with a person with a SSN or TIN and then transferring ownership to yet another without same. Certainly there should be no problem to set up the account with a qualified person now and transfer ownership to grandparent later if they get a TIN or SSN --provided one picks a state with liberal ownership transfer rules.
Note 0% beats negative % of some current 529 performances.