I'd rather just a straight up term of 30 years (or whatever number is most reasonable), regardless of whether or not the author is still alive.
First decade for free, $10 registration for the next year, doubling every year after that, in perpetuity. This allows the "owner" to extract any economic value they can see in the item, and very quickly puts the vast majority of works into the public domain. Central registration also makes it easy to find the owner if you actually do want access to the work for licensing or the like, or to find out if the work has been registered. Each ten years the cost go up by a factor of 1024.
Year 11 - $10 (total $10)
Year 12 - $20 (total $30)
Year 13 - $40 (total $70)
Year 14 - $80 (total $150)
etc.
Price for year "n" = $10 x 2^(n-10)
Total price to pay for every year up to and including year "n" = $10 x ( 2^(n-9) - 1)
Year 20 costs $10240, total cost $20470
Year 30 costs $10,485,760, total costs $20,971,510
The details of the free period length or the first yearly amount can of course be changed, but the doubling rate is what makes this type of system work. Make it five years free and one dollar for the 6th year, and it works great too. Heck, one penny for the first year gets you to the ten bucks level in a decade, so maybe that's the way to go.