Hokkaido isn't as worthless as one might think. Hokkaido contains 1/4th of Japan's arable land, has relatively large reserves of Iron and Coal, modest reserves of Gold and Copper and minor quantities of Oil, in addition to rich fishing grounds. It is also inhabited by the Ainu, who wouldn't be that hard to subdue. It also has the added benefit that the Japanese also did not control it until the mid 19th century. The problem is why would a European nation want to settle Hokkaido in the first place and how it would protect it from Japan? Trade with the Japanese might encourage controlling the Island, but the main candidates in that case would be the Spanish, Dutch or the Portuguese would be easy prey to any Imperialist Japanese State. Thats assuming Trade was important enough for the establishment of trade settlements on Hokkaido in the first place; to do that I say you'd have to have a POD stop the isolation of the Edo period or have a much more Christian Japan so that trade relations would be more cordial and thus profitable. Even then, its very likely they would not bother. Maybe the Germans or even Italy could pick it up in their scramble for colonies but not very likely. As for Sakhalin I don't see how any nation would want it other than the Russians, and having it would likely mean constant conflict with the Russians. Sakhalin just isn't worth the effort.
Alternatively have a Dutch, Portuguese or Spanish explorer chart the island and run into some gold which could spark a lust for gold, even though there is only a modest amount on the island. Problem is, the most likely way of finding this gold is by running into some natives with golden jewelry and I am unsure if the Ainu had jewelry like that.
