My interpretation is that the key thing is if the ATC unit uses FMC Waypoint Reporting (FMC WPR) or not. If they do, then any kind of waypoint in the FMC will be transmitted as part of the position reporting process. If FMC WPR is not used, then you can have as many user-defined waypoints as you like.
I use LIDO charts and the FMC WPR status of an ATC unit can be easily checked in the same place that you would normally check for the CPDLC logon code.
Off the top of my head Auckland Oceanic, Gander and Shanwick use FMC WPR. The number of units that use it have reduced, as it is cumbersome since position reporting data is sent to the operator first, which in turn is then forwarded to the ATC unit.