Does RAM SS have any means for implementing lateral spring supports in a model? Or a method to account for passive soil pressures for a structure with unbalanced soil loads?
You can apply elastic foundation springs in RAM Frame - Analysis mode (Assign menu). Lateral loads due to earth pressure can only be entered as story forces or lateral nodal loads.
You can apply lateral loads as pressure "other" loads on lateral walls and create custom load combinations too, right?
Yes, pressure loads on walls were added when we implemented tilt-up design a couple of versions ago. Those can be used for earth pressure against any lateral wall.