Hi ueda,
This approach might help you.
Use a global var (i.e. currentPage) to be saving the value of current multipage page:
var currentPage; //add this on custom.js file
On the onPage Changing action you will set the value of this var, everytime that the page is changed:
currentPage = $(newPage).attr("id");
To make default a page, associate next code to a button in a multipage page:
localStorage.setItem("default", currentPage);
Then in the phoneui.documentReadyHandler function in your _custom.js file you pull this default var and navigate inmediately to the last page that was set as default:
var default_mpp = localStorage.getItem("default"); //get id of saved default page
if(default_mpp) // check if there's already saved an id, if not, is not necesary to do the transition
phoneui.gotoMultiPagePage('m1-mptest-multiPage1', 'SET_PAGE', default_mpp, 'NONE'); //you should change the first parameter with the name of your project and multipage widget id: 'm1-<project>-<multipageWidgetID>