For the TStrings/ TStringList you would have to subclass them so you have your own TNxStrings class and thus can create an extended version of it. Personally i would overload the Add() function.
So have TNxStrings.Lines.Add(txt) and TNxStrings.Lines.Add(fmt, [parms]);
The subclassing is something you'll have to do anyway to support widestrings too.
For the other controls, most editors would benefit from an public AsFormat function to set the Text property.
Btw glad you like the idea.
It look like good idea. I am not yet sure where I can add it, but first I have in mind that it may fit well for TCell object.
I think that I will add it soon.
G.W. van der Vegt