What you were trying to suggest was essentially a problem because there was no way to differentiate between the story and the stage directions after awhile. For example, allow me to extract a sample from an MST I'm working on at the moment and do it the way you suggested.
"Oh, boy", said Pichu.
Xanthine: I'm going to say something witty here.
Oh, but I have a weakness for electricity,
Syntax: So am I.
Eevee thought for a second, before she took her fighting pose.
Andy: Me too!
Note that in the above example, the italics are essentially reversed. What's italicized in the original work is stripped of italics, and what isn't is italicized. Also, note that even with the bold, what was originally italicized now blends in with the rest of the text. It would be the same effect if you did something like this:
"Oh, boy",
Xanthine: I'm going to say something witty here.
said Pichu.
Oh, but I have a weakness for electricity, Eevee thought
Syntax: So am I.
for a second, before she took her fighting pose.
Andy: Me too!
Or this:
"Oh, boy",
Xanthine: I'm going to say something witty here.
said Pichu.
Oh, but I have a weakness for electricity,
Syntax: So am I.
Eevee thought for a second, before she took her fighting pose.
Andy: Me too!
However, if we change the font face the way Thesis did (regardless of where he got his idea), what you get is this:
"Oh, boy", said Pichu.
Xanthine: I'm going to say something witty here.
Oh, but I have a weakness for electricity,
Syntax: So am I.
Eevee thought for a second, before she took her fighting pose.
Andy: Me too!
Note that by differentiating the font faces, you now have the option of changing the basic styles -- italics, bold, underline, whatever you want. The reason why is because now, the basic difference between the actual story and the witty comments are already made as clear as day with different fonts. Therefore, it's not possible to confuse the story's italics with the witty comments' italics, and you can easily code whatever the author originally intended, rather than strip away all formatting for the sake of making it clear you're still on the story.
Long story short, the only reason why I'm not going with your suggestion about reverse italics and nothing else is because it blends in with the witty commentary. And then, if I changed the font face and then did reverse italics, the font change itself would defeat the purpose of doing reverse italics in the first place. Thanks for the suggestion, though.