IMO, you should not scan for return codes, but for system messages. We do not use a "joblog formatted in a special way" as you state above, but our tests system even just logs the out-of.the-box system messages. We do not run installation exits and/or job log formatter. I know many z/SO installation do have stuff like that, but they all vary, and a generalized tool like this plugin shouldn't rely on anything but the plain system message.
My proposal: Scan only SYSMSGLG for IBM system messages IEF404I, IEF450I, IEF451I, IEF453I. I started to describe and discuss this approach on http://ibmmainframes.com/viewtopic.php?p=330349, and there is yet a better idea to be found.