well, I tried that and all I can say is hindsight is the best perspective.
If I did it again. I would deliver the car with all of the holes plugged really firmly. And then ask the dude to not spray the holes.
A little sand (garnet, beads, soda...what have you) may get in...however it will save a hell of a head ache later.
Either way despite the head ache I would definatly sand (garnet, beads, soda...what have you ) blast the car again before beggining rust repair. Pays dividends later in the project when you begin to discover more and more rust.