I am still not completely sure if there is any functionality we would need to add to achieve this.
Using the top and bottom raster for one layer, this class will create a completely bounded (i.e. waterproof) piecewise linear complex and write this to a TetGen smesh-file. 
Once you have the smesh file you can use TetGen to create a constrained Delaunay tetrahedralisation of the PLC. This *includes* subdividing the layer in off-raster-direction if you set the TetGen parameters accordingly. (For instance, you can specify an edge-radius ratio for the resulting Tet-elements, so TetGen will automatically subdivide a layer if this ratio cannot be guaranteed otherwise.)
Is this what you mean?

In a later PR we could add a dialogue for setting all the necessary parameters such that TetGen need not be called separately after writing the smesh file. However, this would require to add TetGen to OGS somehow as you can only download the source code. *if* we do this, we could skip writing the smesh file completely and include TetGen as a library. The TetGen-cmake-package already provides for that.