Hi Manuel, the spec actually allows for this (section 8.2.4):
A Resource is Executed by sending a CoAP POST to the corresponding path. The request MAY include the value (e.g. as a String) in the payload reresenting the arguments of the “Execute” operation. Note that the behaviour of the “Execute” operation, whether it uses arguments and how those are interpreted, and how it returns values is specified in the Resource description of the Object.