For now we can use some tricks since Master pages behave like Symbols in Designer. You need to: open the Layers panel, unlock the "master_page_name" corresponding to your current page (there's one for each page you applied your master page to), select the frames you want to modify and open the Symbols panel [detach] or un[Sync] before modifying the frames. It's not easy and natural, and I'll need to do it more that twice to be at ease doing it… I hope they'll improve it