Thanks for your answer. I've actually had thought of that already and it works, but it's a very convoluted way about doing a pretty common operation. Maybe someone should tell the Inkscape guys to add a "Select Mask Object" feature?

Maybe I'm misunderstanding something, but the way I'm doing it now, masks appear not that useful to me, because as soon as an object is masked (select object, shift-select mask, "Set Mask"), it is impossible to edit the mask object anymore, apart from its shape using the Node tool. But for...