I guess I sorta know what you mean on something like "About A Girl" but seeing as how he was trying to sound like The Beatles on that one, I just sing it like them instead. Ditto with "Been A Son" and "Drain You", 2 other fairly Beatleseque Nirvana cuts.
I actually did a sexy taco-themed parody of U2's "One" for a ladyfriend where I tried to imitate Bono. Came out pretty good even if I wound up sounding nothing like him.
Beach Boys songs are pretty hard to sing in general. I can do "Warmth of the Sun", "In My Room" and "You Still Believe In Me" pretty well but that's about it. Those falsettos are brutal.