I think by framing devices, you're simply saying the story is told by a character who never appears in the story. Versus normal third person, where the story is told by a body-less entity. 

Some examples:

3rd Person:

[i]"the world was in peril."[/i]

[i]"Jack picked up a coin. He wondered how it got there."[/i]

what you want:

[i]"our world was in peril."[/i]

[i] "Jack picked up a coin. How do you suppose it got there?"[/i]

I thought this approach was called "4th narrative" but i can't find any evidence on google.

I think the main problem with framed stories for RPs is that it usually requires a group of people to share the same character (the narrator) which can cause drama. You could split up the narrators, but then why not just have everyone write in first person if you wanted to flavor the posts with some personality? 

I suppose it's a matter of taste. You could do it, but would the pay off be worth it?