You will have to test it out. First test with a simple cube on each land region. Am not too familiar with spline but should be possible.
I would use a strucutre LandStructure that would contain:
- Enum LandName
- Enum OwnerName
(the structure can be used as saving progress to the game instance)
From your BP_Land, EventBegin load the strucutre.
Switch on OwnerName:
- NoOne > Keep Hidden (can be default)
- Player X > Show mesh, Set material parameter colours of the mesh spline and show owner name on map
the enum landname can be used as an id for lookup purposes