To know the answer, you need to know the architecture.
First, there are 2 major parts - the runtime (server side) and the editor (powered by the server but runs client side)
You edit the configuration in the browser - but when you deploy, the actual runtime on the server gets updated. Thats where code like the serial port runs.
So in short, no, you cannot simply poke values into the browsers console and affect things like the serial port.
Typically, you use debug nodes at the entry and exit of a node to see how it transformed the msg object.