Hi wim, for the isoparm method that does work on only a single face at a time. To make that convenient on something like you've got you would want to run the Rebuild command on your curves before constructing a surface from them:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild

The Rebuild will get rid of segmentation of your curve, it makes one large uninterrupted segment out of all the smoothly connecting sections of the input curves. Then when you construct a surface out of the non-segmented curve you'll get a big single surface instead of segmented faces.

It is only for doing a sort of "local to the surface" horizontal or vertical direction line though, but that should pretty well naturally follow the surface structure from how the surface was created though.

- Michael