there might be a better approach, but i was able to substitute a custom locator successfully using this approach..near line 2697 of main.js
//define the custom locator
var geocoders = [{
url: "http://tasks.arcgisonline.com/ArcGIS/rest/services/Locators/TA_Address_NA_10/GeocodeServer",
name: "NA Geocoder",
//substitute whatever parameter your own locator uses for single line input here
singleLineFieldName: "SingleLine"
}];
_self._geocoder = new esri.dijit.Geocoder({
map: _self.map,
//setting arcgisGeocoder to false ensures that the url specifed for "_self.options.locatorserviceurl" in line 295 won't be used
arcgisGeocoder: false,
theme: 'modernGrey',
//pass in the locator (and options) you defined above
geocoders: geocoders,
autoComplete: true
}, dom.byId("geocoderSearch"));