WHILE members of the Group of Seven (G-7) industrialized nations met last week in Bangkok to discuss aid to the Soviet Union, the United States remained interested in clearing the way for more Soviet aid from fellow G-7 member Japan. Secretary of State James Baker recently urged the Soviet Union to return the Kurile Islands in order to end a territorial dispute that has long soured its relations with Tokyo.One wonders if Mr. Baker also mentioned southern Sakhalin Island, the other piece of Japanese territory seized at the end of World War II. Since Moscow desperately needs Japan's help, why not provide Tokyo with some real incentive by also returning southern Sakhalin? The US has a long history of intimate involvement with the issue of Sakhalin Island, known as Karafuto in Japan. The 1905 Treaty of Portsmouth (N.H.) that ended the Russo-Japanese War awarded to Japan that part of the island south of the 50th parallel. Forty years after one Roosevelt implored Czar Nicholas II to part with this piece of land, another agreed at Yalta to allow Stalin to take it back. According to the State Department, the official US position on Sakhalin and the Kuriles essentially has not changed since 1951, when the US declared that the fate of these territories must be determined by future "international solvents." Although Tokyo is not so deliberately vague, the essence of its official position on southern Sakhalin is the same - ownership is a matter to be decided by "international law." While not pressing the Japanese claim at present, neither Tokyo nor Washington regards the clause on Sakhalin in the 1951 San Francisco Peace Treaty as binding on Japan. John Foster Dulles stated at the time that since the Soviets did not sign this treaty, they had no legal right to southern Sakhalin. Andrei Gromyko refused to sign the treaty, partly because it did not specify to whom Japan was relinquishing its claim. During Mikhail Gorbachev's dramatically unsuccessful April appeal for aid in Tokyo, he did not budge on the question of the Kurile Islands. After the August coup attempt, Moscow has seemed more flexible - even announcing this week a reduction in its military presence in the Kuriles - but not on Sakhalin. Last month, Grigory Yavlinsky, an economist attached to the new State Council, suggested that an 1855 treaty between Russia and Japan be used as the "moral and legal starting-point" in solving the Kurile dispute. This same treaty states that Sakhalin is to remain "unpartitioned." Mr. Yavlinskiy added that the 1855 pact was an agreement "that the two countries concluded after friendly negotiations and did not result from a war," a clear reference to the 1905 treaty that gave part of Sakhalin to Japan. Despite the fact that Yavlinsky is no diplomat, he did not speak out of turn, as some suggested. His carefully chosen comments, made in a written response to questions from a Japanese news service, indicate that on a critical issue that demands bold and imaginative thinking, Moscow will be capable of neither. The Russian Republic alone is seeking "billions, not hundreds of millions" from Japan, according to a top Russian legislator who visited Tokyo recently. While Mr. Gorbachev has indicated that he would like the territorial dispute resolved by mid-November, the next move is Moscow's. Japan pledged $2.5 billion in emergency aid for the Soviets last week. At winter's end, Soviet republics will again be at Japan's door, seeking longer-term aid, credits, and investment. Under these circumstances, it is only na tural that Tokyo ask, "what's in it for us?" How will Moscow respond if the US asks the same question?