Managed to solve this one by myself;
In order to call a C++ function from python, it must be implemented as a BlueprintCallable.
You cannot implement a function as both BlueprintCallable and BlueprintImplementableEvent for obvious reasons.
(though i did try anyway, and no, it did not work)
So, the solution is to make another C++ function with UFUNCTION(BlueprintCallable), with the sole purpose of calling your BlueprintImplementableEvent.
.h
.cpp
Because of this behaviour, I wonder if using BlueprintImplementableEvent would allow you to overload a C++ function in python…? Interesting, might look into it later.