Ooh, this is interesting! It doesn't "lock" the same way the securely unsecured format does because there's no insert to keep the tab from slipping out of the zine, but if your cardstock is robust enough it'll pretty much stay where you want it. I'm gonna have to give this a shot!
Yay! The cover on the example is watercolour paper, but I forget what the gsm is, alas.
The securely unsecured format is a lot like one I learned that uses multiple tabs (through signatures) that lock in the same way (insert/bookmark) from a concertina inserted through a slit in the cover spine; the outside spine tab/flaps of the concertina strip could be left free, or folded and glued down. (and I was sure I had an example somewhere ... >.>;;))