Butterfly Knots on Backup Rope

This is totally old-school, but you can use butterfly knots set about 5-10' apart on a rope as either your backup or even as your primary protection for toprope solo. Just put a bunch of them along the rope you'll be using, make sure they're tied well, and off you go. You clip them just as you would clip a bolt as you ascend. It will make sense if you give it a shot.