But, if you are in Revit 2019. It appears that it is available in the Revit 2019.1 API additions.
Revit Link Additions
The new function:
RevitLinkType.GetPhaseMap()
allows users to read the Phase Map parameter for the link. The phase map is a correspondence between phases in the host document and phases in the linked document, and is used by room calculations. Note that the link must be loaded to read the phase map.