Nothing more to be said, really.
This has come up before - it’s not possible. The OS X text system allows temporary attributes (formatting) only for colour, not for character shape/font. So it’s possible to temporarily change the text colour and nothing else. (Note that programs that allow a different font in full screen are plain text not rich text, so store no font information.) I don’t think this is a deficiency of the text system in this case, either, as it makes sense - showing one font where there may be a variety of fonts beneath (each with different variations - some allow italics, some do not etc) would be hellish not only for the text system to manage but also for the user in knowing whether or not text changes will change the underlying fonts that are hidden.
Yep, see the problem. Fair enough.