FULL BEAR TRAP!
"You can ignore my great advice but I do not recommend it (look at my scars)!"--Summer and Eve
John Romita definitely did. He worked on Young Love (and possibly other romance books), prior to going to Marvel and Spider-Man.
I *think* Cardy did some cover paintings for Marvel's magazine division.
I don't know about sequentials, but Sienkiewicz did a Superman pin-up for Superman #400.
And I see I repeated myself in this thread... See kids, this is what happens from starting at the beginning of an old thread you don't even remember.
Did Chris Claremont ever write anything for DC?
Pulling for: HATE!; LXG; Doktor Sleepless; S.H.I.E.L.D.; Batman, Incorporated; X-Factor;All-Star Western; Sergio Aragones Funnies; Saucer Country; The Manhattan Projects; Secret
Also Gen-13, and the classic Justice League run that introduced Crucifer the Vampire to the world.
Crucifer the Vampire.
MarkAndrew at Comics Should Be Good
How about George Papp? Any Marvel work for this longtime toiler in the DC vineyards, notably on Superboy and Green Arrow.