Well, I hope it is not a limitation problem pf the RAW, since setting downtime is possible with the HTTP API up to 2.0.0.p5, which I am currently using. Hope it is just s bug.
It doesn’t make a sense sending API requests to the respective slave for these operations, I always thought using the API for some actions is a as good as using WATO as long as the API supports it, so this shouldn’t be a new limitation of the RAW. But who knows.
Thanks @andreas-doehler