Not that I know of. I've never heard of them handing out free ponchos (I know you can buy them if you want to).
But I heard that AK installed a special section of lockers just outside Kali that are free for one or two hours; you can store your stuff while you ride so it won't get wet. It's not an all-day locker like you can rent at the front of the park, but it's nice to be able to stash a backpack if you're riding, without being charged.