In episode 337, Robin is shown using her Demon Fruit ability to save Usopp when he is hunting octopus in the aquarium of the Thousand Sunny, but I can't remember, whether it has ever been used in a similar manner, so I assume it's possibly an exceptional case.
Was it shown at any other points of the series if she could do it?
Although there may be others, so far I've only been able to think of one other time where she used her powers in a similar manner. The picture below is taken from the Strawhats' trip to the Fishman Islands (Chapter 605). The ship is coated with the resin from the Yakuriman Mangroves in order to withstand the very large hydrodynamic pressure. There is therefore a "impassable but transparent obstacle" as the OP puts it between Robin and her use of her DF power.
