I'd still suggest using getChildByName rather than this['hotspot'+i]. You're guaranteed that the thing you get back is a DisplayObject (and therefor has an alpha property), and also that it's actually on the stage. I'd further suggest that if this will be called repeatedly that you collect the hotspots in a proper array so that you don't have to go through the whole displayList over and over.

Are the hotspots guaranteed to be direct children of the object calling this code? If not, you'll have to write a recursive function to collect them all.