Hi Meyrick - If it is really simple, like pegs and holes, you can probably get away with primitives - Cylinder for instance, to create the added parts, But combining them with the parent mesh may or may not turn out to be a chore. Basically I think, you’d make your splitting planes with the pegs sticking out on one side and MeshBooleanSplit the overall mesh with these.
After MeshBooleanSplit:
The problems may arise in the mesh Boolean part- these are not always ultra reliable in Rhino… start by making sure the mesh is clean -Check will tell you about some potential problems (not uncommon with scans)
-Pascal