Instead of defining a template for the groupHeader, you can define a function to call. Inside that function you can whatever you need and return whatever you want for the header to say.
headerTemplate: headerTemplateFunction,
with the function like
function headerTemplateFunction(e) {
var matchedData = $("#listView").data("kendoMobileListView").dataSource.data().toJSON().filter(function(item) {
return item.continent === e.value;
});
return e.value + " has a total of : " + matchedData.length;
}
See sample on jsBin http://jsbin.com/EdiPEVo/2/edit