We flew on CM EWR-PTY-UIO, and were able to secure Exit Row seats at check-in on 3/4 of the flights. In fact, exit rows seem to go unmanned. We were the only pax in Rows 12/14 on two of the flights. Just check in reasonably early, and unless the flight is packed you'll have a chance at it.
That said, if you're CM/CO elite, I think you can reserve them ahead of time. Now that NWA Elites seem to be getting upgrades, maybe NWA Elites can pre-reserve them, too?? Not sure.....