I 3d print my block off or masking parts. Cheap PLA is pretty good as masking material in the cabinet. While I own a blast cabinet I tend to use it for small parts or quick jobs, larger jobs go out to get done. Problem with sending it out is that you run the risk someone blasts an area to hard and damages the part geometry on critical surfaces. It's happened to me before were the local shop blasted some motorcycle rims so hard that the disc brake mounting surface goes out of true. If the part shows up to the shop with block off plates it's pretty good chance the critical surfaces won't get damaged. On aircraft engine parts I block off the entire internal cavity of the engine on all parts before blasting. There have been reports that blasting media can get imbedded in the surface of an engine only to fall out in use and damage the engine. How true the report is I don't know but have zero interest in finding out. I do think your idea is neat.
