The thing is, he didn't need to be in the movies _per se_ --- they just needed to have the Hobbits swallowed up by Old Man Willow, then fade to black, then they

The thing is, he didn't need to be in the movies _per se_ --- they just needed to have the Hobbits swallowed up by Old Man Willow, then fade to black, then they awaken on the Barrow w/ a couple of swords, and the ponies tied up nearby, and a figure riding away on a pony humming/singing a tune. Folks who had read the books would know what had happened, folks who hadn't would just have to accept it as a magical incident/rescue w/o explanation.