Hi Kilian! Today is a full day for you :)
I'm glad to announce that REST expose PATCH capability is available on the last Platform Version 11.12.0. On this version, the capability is being released in Technical Preview and to take advantage of it needs to be enabled through lifetime.
To manage expectations, this version is the starting point to allow expose REST services using the PATCH verb. However, the platform at this stage is not pointing out which were the attributes that were sent or not by the requestor which we are aware that is not. To get more details on the delivered capability please check the documentation page.
Please provide feedback using Service Studio submit feedback.
Please, keep the feedback and new ideas coming!