Shellac should stick to and seal the urethane. Shellac is not waterproof, so if you want to use it to seal the tacky urethane, you will need to coat it with something else. I would suggest an epoxy formulated as a finish coat, for example West System 207 hardener. Note that water is insidious. If you get water on the trim, it will work its way through the smallest cracks, joints, and nail holes and attack the oak from the back, end grain, etc. To keep the wood from rotting, discoloring, shrinking and expanding, it is necessary to coat it on all surfaces with an unbroken coat of the finish.