I don't know about the ROC, but the PRC was bitterly opposed to the return of Okinawa to Japanese authority (until the US and Japan made overtures in 1972, and there was the prospect of an anti-Soviet pact). If I recall correctly, they kept making demands that Okinawa should be made independent.
But I don't think they have much in the way of an actual territorial claim, any more than they would have had over, say Korea. Both were tributaries of China, after all, but Okinawa had dual tributary ties, with Japan and China. I don't think the Okinawans themselves would take any more kindly to being part of China than to being part of Japan, especially since ties to Japan - even before their annexation - were generally stronger than those with China.
I reckon they'd be squarely refused by the US. With a more violent Battle of Okinawa, and more oppressive rule by the Japanese before the battle, you might get more pro-independence activities from the Ryukyuans. If the Republic of China can stamp its authority on Taiwan more firmly after the war - and beat the Communists - then it might be more proactive in securing the various islands near Taiwan. But I doubt that they'd be able to lay a claim as far as Ryukyu.