I am wondering if it is possible to customize the default InfoWindow that pops up when the user clicks on various businesses that Google displays on its default map (see screenie). I would like to add functionality to let people "pin" these locations if they have stumbled on them while just browsing the map.

It is not currently possible to listen to any click event on the POI icons so overriding/modifying the info window is not possible.
File a feature request asking for it.