Does your seat post have a quick release?
If so, you could remove the seat post, run the post through the cable loops, and re-install it into the frame (assumes you've marked your height so it's returned to its proper location).
If not, and if you have drop bars and it's just a short quick ride to the store/wherever, sometimes I've just run the loops of the cable up the handlebars (careful not to interfere with the brakes). I tend to ride in the drops, and that holds it in place.
Or unlock it, wrap it around the top tube, and relock.