If there are enough requests for this, we will document and advertise how to use it. We want to keep a clean separation so that VGUI is aimed purely at the structural aspects. Also, such attributes become language-specific. It could hinder the commonality as a general structure capture utility. We are continuing to investigate this and plan to address this in the future.
Specific suggestions are welcome.
For now, you have to open your text editor as you normally would, from your text window or file-browser. However, there are several logistic issues that we are working to solve. Several models may be defined in a given file. The user may attempt to open the text-editor on a file that is already openned. If a model is not yet defined, should we open a text-editor to a new file, or add it to an existing file.