I've been diggin around trying to find an option for creating a totally empty node that I can fill in with my own custom attributes. The closest thing I've found so far are the utility nodes, but obviously this isn't quite what I'm looking for.
I've never needed to do this myself and so I have no idea, BUT, depending on what you are doing, how about creating a empty group node which you can add your attributes too? simply use the -em flag in the group command, i.e.
I had this same issue a couple weeks back. I was looking for a node to put in my scene that was essentially empty. Group node was my first thought, but I had no need for a transform so I kept looking.
There lots of node types in Maya, but I was looking for one that had very few other attributes, so when I had it open in the AE, it would be mostly empty aside from my extra attributes.
Unknown fits this bill, but it made me a bit uncomfortable to use a node that was primarily for handling undefined nodes. It is hard to put your trust in the unknown.
I ended up going with with a materialInfo node, which is a very empty node type and seemed to make sense as my work was shader related.