Frank Zappa comes to mind
Basically only these two guys, but honestly neither one of them are truly accepted in the classical world. It would be extremely rare that you would see Zappa on the same program as Beethoven.
Yeah, even though I posted a pic of Greenwood, I know he isn't respected. I don't think Zappa quite makes the cut either. Maybe there's someone though. I'm considering performers too, not just composers
It's useless to imagine "pop music" before it made sense as a concept, but I'm tempted to say Liszt is the closest to fitting this bill, Lisztomania and all
Lang Lang played with Metallica at the Grammys