Finished it today during my lunch break! Fully stretched out, it's not quite a foot long (well, we have some big ol' books!).
I tatted the tail as lock stitch - the equivalent of 30 DS, then did one ring with each shuttle, and finished with a SR. I continued with more lock stitch, same as before, then did three SRs in a row. One more section of lock stitch chain finished the tail; then a not-quite-closed SR to hold the tassel, and the bookmark was finished!
The thread is size 20; Berry Burst (Lizbeth) and the lavender I was using a couple of posts ago (DMC Cebelia).
If anybody has a sure-fire method of making tassels, I'd love to hear it!