Aesthetically, i don't think i can get behind "big rock," Clive. lol
Without changing the fundamentals of the scene in any way, or how the ride itself operates, wouldn't the easiest thing to do be changing the Ultrasaur's motion in general?
Have it always standing tall, neck elongated - mouth chewing on plants hanging from its mouth. Mouth has a circular rotation of the lower jaw animation always going - doesn't have to be anything too dynamic, just a slow rotation joint. The top 20% of the neck sways gently with the chewing to stop people from thinking the entire thing is static from when the doors open. Then, only when the boat approaches or when the boat passes the trigger for movement, it lowers the other 80% of its neck to crane at the passing boat. Play a smaller sound of chewing and groaning as you get closer, and play the same kinda "moo" we already get when it passes by.
The neck-breaking reveal is already awkward, just take it the other 50% of the way it needs to turn it into the big cow that it already is: It stand. It chew. It look. It moo. lol