In the examples I found so far, CODESYS was used for building demonstrations on a PC (e.g. using the simulated PLC) or for industrial PCs (not PLCs).
Is there a way to export/upload a CODESYS project to proprietary PLCs (AutomationDirect, Allen Bradley, etc.)?
You need a PLC that supports Codesys runtime. So it's not possible to upload a Codesys project to PLC, that is not made for running Codesys based code.
There are many Codesys PLCs that aren't PC-based and they are used a lot for example in mobile machine control system. There is some kind of a list available at Codesys website. (Select device type from tree-view)
Many companies also build their own programming environment and it's based on Codesys platform, such as Beckhoff (TwinCAT) and Bosch Rexroth (IndraControl V). Therefore it's easy to export/import code that is made in original Codesys programming environment. Many files work straight and same thing is with libraries, they often work 100 %.
It's also possible to export/import code in PLCopen XML format, which should be nowadays quite wide used even with Omron/Siemens/etc. environments.
So shortly: It's not often possible to upload the Codesys project to other brand easily. If the other brand is not Codesys based, it's even harder. But thanks to that PLCopen XML, it could be possible, at least block by block. And there are hard Codesys PLCs that aren't PC based - a lot.