The way that I understand it is that in short, you can't, since the persistent overlay is a copy on write table, that only really works by marking changed blocks from what it overlays. Hence the size limit, the corruption when it becomes full and the uselessness unless you boot into that system. If I am wrong I would be glad to be corrected, because until these issues get fixed I am sticking with live distributions that use debian-live, which uses aufs, which doesn't corrupt is unlimited in size and can be mounted out of context. But these features won't come to fedora until a union-mount file system makes it into the mainline kernel (or so I have heard).
Good Luck,
Alexander