Can I insulate under my condo's subfloor?
You can easily unscrew off cover of "dummy register" and see if there is a duct on them. No matter what they are a doubt they are big enough to fit through.
Most of the time an access hole is in the floor of a closet or utility room. If you dont have one I would create one there. Look hard though you might find an opening somewhere. If there is ducts down there probably also plumbing. The plumber had to get down there after the house was built to install them.   There is probably a foundation wall and box/floor joist in the way from the garage or anywhere else outside unless an openning was created when house was built.