This project may be interesting. It talks directly the Omron proprietary FINS protocol: https://github.com/jmor2000/LV-OMRON-FINS
No OPC, no special Ethernet/IP function block or whatever needed on the Omron PLC. While the Omron FINS protocol is proprietary it is actually openly documented by Omron: https://www.myomron.com/downloads/1.Manuals/Networks/W227E12_FINS_Commands_Reference_Manual.pdf
I was working with a PXI-8840 dual-core, so slightly different than OP's, but we were successful in replacing the RAM using off the shelf parts just this past week.
The original 4GB stick it came with was this: https://www.crucial.com/memory/ddr3/ct51264bf160bj
We just bought the exact same model*, with instead 8gb: https://www.crucial.com/memory/ddr3/ct102464bf160b
There wasn't any issues installing it, or being recognized.