I think your request is overly constrained. Although I'm a big fan of double-duty gear, I think in this case you need one of each.
If was easy to find, I wouldn't have posted a query. I disagree wholeheartedly that this should be an impossible quest. I think it entirely feasible, and am surprised a manufacturer hasn't produced pants to fill this need. None of my tours have been done taking one of each. Had a pair of Performance Gore-Tex rain pants that passed, albeit barely, as casual pants. They are not longer available, and what Performance sells now isn't close. Had one manufacturer make me a pair; unfortunately he made them look like sweat pants. He agreed it is a doable project; he just couldn't pull it off. I expect a perusal of this forum will unearth quests that are lot more fantastic than wanting bicycle rain pants that can double as casual pants.