You need to think it outside of box, in blueprint editor technically nodes are only UI elements. are same as code text, all they are is mainly UI data, they only track what is connected to what and based out of that data it build VM code. So you can do anything that Slate and Graph Editor widget (which is not only used in Blueprint) allows… so practically anything. UK2Nodes are only used in editor once they compiled inrmation about them in the VM is destroyed as it not really needed to execute it. It like you have ability to write any text in to C++ and make compiler integrate it whatever you like
So technically you should approach to UK2Node more like to a UI widget class (in fact they are graph editor widget system components) then a code piece that does something. Once you understand that making node without pin should sound trivial for you.
The initial set of pins are set up in AllocateDefaultPins function using CreatePin function, so you should override it and create pins that you need and pins same as nodes are just UI widgets, alone they not much functional. So making pin not appear is as trivial… as doing nothing ;p After that function is executed, you can access pin object and manupate them on varous events in K2Node
And it not just pins K2Node ocntrols all aspects of node appearance, title bar, it color etc and oyu also have option to create your own slate UI in it as node widget all together. that what i’m saying UK2Node is more like a widget then code piece