Ok, answering my own question in case this helps someone searching for the same...

I was able to successfully read a PFPX flightplan into the CDU by normal Export from PFPX to Prepar3d (<Documents>\Prepar3D v4 Files) and then in the CDU reading the flightplan using:
RTE,
[LSK3] REQUEST SEND,
[RSK6] FLTSIM FPLS

The key here is not to try to read the PFPX flightplan directly as a CO ROUTE, but as an IMPORT FS FPL.