A highly creditable legal professional on Forums advised me to list this on the customs form as a "pipe accessory." It went through just fine. I do not know the customs system nor enforcement, but I considered this some highly valuable pro bono advice. As we know, these systems are arbitrary and sometimes ridiculously unkind -- but mostly are not.
Add my own advice, inside the package, carefully label the baggie or tin (or whatever) with the words pipe tobacco and exact blend. If the package were inspected, this might allay suspicions that it is some kind of hard drug or some kind of agricultural contraband.