I should have been clearer with the subject of this post. It's not the "icon" per se that I want to change, but the functionality available in the action window. Judging from the lack of responses to this, I'd guess that customizing the action window is not a feature other people are looking for.
On a somewhat related note, does anyone know if it is possible to add additional "unlisted" commands to the Menubar/Toolbar? I'd like to add a command, for example, that would search for media servers, but it is not listed in the menubar customization choices. If you are wondering why I'd like to do that, it is because I frequently switch between the default library (in order to make changes to the database), which is located on a central server, and media server (which doesn't permit changes to the database, but reduces my risk of corruption). Hunting through the drop down menus is not overly burdensome, but it would be far easier just to click on an icon on the menubar.