I found something wonderful on Pinterest ... the Ogham alphabet! Here's a link to the site
http://www.celticbritain.net/ogham.htm which is in Dutch, so you'll have to use Google translator if you want to read the description. BUT ...it looks like simple stitches to me and I think it would be a good secret way to sign your quilts! Unfortunately, it is missing a few letters that would be handy, but ... what do you think?