Assign random Mat Instance to Landscapes during actor creation (C++ only?)

I’m using many sets of playable landscapes that will use a random auto material based on climate conditions.
This setting only needs to be applied at the creation of the terrain, therefore not exactly at “runtime”.

I get that material changes are possible for landscapes through a boatload of scalar parameter blueprint nodes, but I’m definitively more interested in assigning individual MI’s, again I don’t need to change these values at runtime.