Copyright is easy to appraise. Estimate the stream of payments it will generate; take the net present value using an appropriate estimate of a safe interest rate.

Will it always match the actual value? No, of course not. Sometimes popularity changes a lot, or interest rates change a lot.

I'm not sure you really need a proprerty tax on copyrights though. They generate taxable income until they expire. It seems more fair to tax the actual income rather than appraised value, to avoid problems from cases where the appraisal is too high or too low.