I'm pretty sure this feature doesn't exist (I just looked), but is there Flash IDE project-wide Actionscript search? The Flash IDE might be a tolerable AS3 code editor if this feature was available, now that code-hinting/intellisense is better in CS5.
Have you already tried to use the Find & Replace Feature? It's not only for replacement but also for searching of multiple types of elements. including actionscript. If you want to search inherited classes then I don't think that will work. Then you should rather switch to Flash Builder or Eclipse..
Yeah, "Find and Replace" would be the logical place for it, but it only applies to the current FLA document, not all classes. I've been using a different 3d party tool for Visual Studio (which has project-wide search), but that has its own funkiness. I've tried trials of FB and Ecllipse plugins too, but I'm not used to working there.