Yea, that would be awesome.
The main problem is that this media query should already exist in the css file, to be visible in the dev tools.
I was talking about addin new media query.
Here is my idea. Not sure if it's very good, but worth mentioning:
Add new media query button. Which will add media query in dev tools, and add togglable menu with it/them.
Selecting some element, then opening that toggled menu, will automatically append #id or .class to that media query.
If user will add some css property in that "temporary" media query, it will automatically add that css rule with media query block into css file and next time it will be visible in default/usual way, like the other css.
Before user haven't added some css property it will not be written to css file.
So basically summary:
- new media query button (with prompt probably or autofocus in toggable media queries menu)
- toggable "temporary media queries" menu
- auto adding #id/.class feature next to temp media query
About usability and UI positioning it's up to designers.