I want to add a functionality to Node-RED editor, which analyzes the current model and performs some actions. For example, configuring an external system based on a flow. I don't want to implement it as a new node type as I think this is not a flow step. I rather want to have it as a button on the top tool bar or as an entry in the menu or somewhere else.
I would be very happy if someone could advice how to proceed.
It is mainly front-end code so as long as what you want to do can be written as a front-end web app, it should be possible. You have ready access to a number of libraries including D3.js and jQuery, jQueryUI plus some others.
The main issue would likely be how robust it would be when Node-RED is updated since, of course, the core development would not take your development into account.