I've read the long interesting discussion about 3d-polygons and TINs:
http://www.nabble.com/convert-3D-polygons-or-Tins-to-a-DEM--td14998420.html
AFAIU, code to manage the creation (and editing?) of 3d features, like
TINs, is still undergoing development.
I think this is a good moment to propose to consider, in the TIN
creation phase, the use of multiple types of input features: points,
lines, polygons.
I come from ArcGIS world, where it's possible to set these as "mass
points", "breaklines" and "hulls" [1]. For example these features are
strongly requested by geologists, when i.e. fault lines has to be
setted, or when a highland is known to "break" the linear
interpolations of points.
Of course, it's a wish... but maybe useful while doing "brainstorming"
about the code to be developed. 
Giovanni