I've only ever seen it work (that is to say, capture my interest) in parody. And in Casira's fabulous body-switch fic, which I'm not sure counts. I read one fic (not in HP) where a guy appeared to be pregnant, but it turned out his body was playing host to this alien... That was pretty cool. :)

But in general...why? Seriously, why? I think that the majority of MPreg writers are people who truly do not get that these men are, well, MEN. It's feminization taken to an extreme.