I think for self-insertion to work in fanfic you can't be as clumsy as original fiction can be, because the field is already populated and you have to work around this. Like my example with the ordinary present day character meeting the SF/Fantasy ones, that is an angle the is not yet taken by the canon characters, so it is easier to pull off.

Ah, that's really an excellent point! I agree. :D And, looking at it this way, I think that part of what makes people react the way they do to self-inserts is the same urge that makes us react badly to characters who are (to us) OOC, or are paired with canon characters we'd never pair them with. That gut-level clench of "It's my character and you're doing it wrong!" - you're, in some sense, taking him/here away from me. (Generic pronouns, of course; I'm not specifically referring to you and me. Damn English and its inadequate pronouns. *g*)

The difference, I guess, is that at least we recognize that ship wars are toxic and that one person's OOC is another person's IC, whereas we're likely to agree across-the-board that self-inserts are "bad" and worthy of mocking. I don't think that I'd realized until reading the above links that it isn't so, and now I'm having to undo a lot of years of fannish conditioning not to think of them as such.