> There are things to sort out, but it's certainly doable. They can probably use direct COGS to determine the tariff basis.
You could construct a variety of accounting methods to calculate a tax on the 'foreign content' of movies, but how do you actually impose this tax?
A tariff on physical goods is easy. A country requires that goods enter the country through customs facilities, and then the nice customs official doesn't release the thing until the tax is paid. The legal and physical ability to impose these taxes is long, long established.
How do you physically impose a tariff on a movie? If the master is transported physically, what is its value? The value of the fixed copy/master doesn't necessarily include the value of the IP involved, in the same way that a DVD of a $500 million movie might have a retail value of $20. What about movies transferred digitally, since there are no customs checkpoints on fiber-optic lines?
What legal apparatus would be used to impose this tax? Trump is currently getting away with the physical-goods tariffs because the legal infrastructure to collect the taxes is already in place, and remaining legal disputes are just about whether the President can unilaterally set or change tariff rates. If you'd need new law to "tariff movies," then the chance of this whim turning into a real tax drops sharply.