Now, I wouldn't say off hand that an attack over the ice from Hiiumaa or the Estonian mainland to Åland would have been impossible in, say, February 1940. It would have been significantly more difficult than the successful Finnish operation to take Suursaari with a similar attack in 1942, though. With Suursaari, we are only talking about crossing c. 40 km of ice. With the operation you are envisioning, the Soviets would have to cross 150 km of ice to reach Mariehamn. Even the ice road between Vaasa and Umeå during the Winter War was shorter than that, and it was a lot more north where they could rely on a stronger ice cover. If the Soviets used ships (with icebreaker support) to bring the troops closer, then there would be a much bigger chance for the Finns to see and understand what is happening.
What we would need, then, would be Soviet troops well versed in winter warfare, led by a highly skilled and lucky, hugely ballsy Soviet commander, one who could sell this highly unorthodox operation to his superiors in the first place. I'd personally rate it unlikely that such an operation would be tried - even if the Soviets had pre-made plans for more aggressive naval operations. And then if it would be launched, the risks presented by the ice conditions, etc, would be significant.
It would make for a pretty awesome story, though.