It sounds that I’m the one misunderstanding. My real-world experience is very much GA as a pilot and Part 135 as a mechanic, so the workings of FMC systems is limited to what I’ve gleaned from flight simming over the last two decades. I know the Garmin 430s I’m used to as both a pilot and mechanic are very limited in their memory, so it only mildly surprises me that FMCs on transport category aircraft have similarly crappy storage capabilities
To clarify, I’d like to enter a company route in the Route page in the CDU, in whichever way that is realistically implemented. From what you’ve said, it sounds like it’s less a stored database on the aircraft than it’s an uplink during preflight. I always wondered since it’s often in the format of MCIMDW01, MCIMDW02, etc.
Whether stored in the FMC or uplinked, I believe PFPX can manage the database of routes when the navdata changes.