There's nothing wrong with 45 objects for detectors - it's hardly extortionate. I don't think there's even much to be gained in exported filesize either. A detector sprite with a solid fill image is effectively just a collision mask in the engine. Plus, you can actually adjust them in the layout view, whereas multiple collision masks would only be accessible via the image editor, which is extra clicks and makes it harder to have an overview of what you have.
So are you sure multiple collision masks is absolutely necessary? To me it's one of those ideas that sounds cool, but upon inspection doesn't actually add much value, at the cost of much complexity.