Thanks for your quick responses Sergio and Victor
It sounds like if I can find a way to produce a 3D DXF using whatever tools I can get my hands on, then the wireframe geometry from that file will appear in Mecway?
Answering my own question, it turns out that indeed a selection set of FACES -- not elements -- may be used to define a named selection, to which a hydrostatic pressure may be applied in the expected direction.