It has come to my attention that Wakkanai Customs (Hokkaido, Japan) is, in most cases,
now requiring a Carnet.
Previously, a simple declaration was sufficient.
I telephoned to find out the details, and the reason they gave me is that many travelers are not able to provide them with their specific plan. Customs wants to know when and where the vehicle is exported. Also, in some cases, travelers have failed to export their vehicles within one year. (Most likely the reason for the change

)
However, they are open to making exceptions in some cases.
If you have an reservation for exporting the vehicle with a shipping agent and can have the shipper tell Wakkanai Customs when and where you will be leaving from, they
might let you in with just a declaration. In any case, it seems advance permission is required. Without a Carnet or advance permission, you will not be able to purchase a ferry ticket.
I will post any updates if I get any new information.
In any case, this year (2015) is possibly the last year for this ferry route. It is not proving profitable, and Wakkanai City doesn't want to provided heavy subsidies to keep it going.