Currently I did only read your post to exactly this point and I felt I need to respond immediately. I do like the idea, because of the working demos and it seems you already use it.
I think we will definitely need some support from the core, especially because there are default core fields that are hidden by default, like modified and so on. Messing with them, has potential to cause confusion.
For my field-editor plugin I did create a "Warnign .. " box, that is similar to the “Existing tiddler title” warning. So users, that want to create field names, that are reserved for the core, or do already exist in a tiddler, get this dialogue.
@jeremyruston – IMO we should implement such a warning dialogue in the core. What do you think. (I’ll create a GH issue soon)