Over the years the rules for carrying fare paying pax in cargo aircraft has been modifed to the point that if you follow FAA/EU rules there are so many requirements and restrictions that it is not really economic.
Apparently it is still feasible in certain markets. Alaska Air runs a bunch of 737 Combis in Alaska, and Asiana runs a bunch of 747 Combis (though I don't know on what routes).
Granted, though, Asiana doesn't have to deal with the FAA or EU on Asia-only routes, and Alaska is a hotbed of FAA waivers and special rules...