Try using the following:
I think this should get you the grid wrapper
var gridNode=document.querySelector(".mx-name-"+ this.targetGridName);
Then you can use dijit.registry.byNode
var grid= dijit.registry.byNode(gridNode);
Once you have the grid you should be able to access the data.
grid._dataSource._pageObjs should hold the objects of the grid
Looks like Jason has this covered. You also might look at the code in a few of my widgets in the app store that do filtering like this: