The simplest thing I can think off is to get a large enough hose and a Y-connector. Run a hose from the output of the Y to the hole. Attach an L-shaped elbow to the end of the output so that it blows sideways and not into the ceiling. Attach an air compressor so that it blows into one of the inputs. Feed pellets to the other input (ideally from the top to utilize gravity). The air stream created by the compressor should grab the pellets and carry them into the target space.
However the pellets will only fly so far depending on the strength of the compressor - this would be a problem with any injector. They will pile up around the hole, and might not get easily to corners farthest from the hole. I imagine if someone planned to use this method they would expect to have several holes, at least 1-2 meters apart. As far as working with what you have, you could periodically plug the pellet input, and let the compressor simply blow air, to push the pellets away from the hole.
If you run into airflow problems, use a thinner hose. It should speed up the airstream. Obviously, using pellets made of a lighter material will work better.
Will you ever need to remove them, though? That might get tricky if you can only do so through your one hole. You'd need a long hose for a vacuum cleaner that can be snaked into every part of the space.