Did a simple copy and paste from MoI, of an extruded polygon, into Alibre, as a bunch of 3d surfaces, including the face for which the acreage is needed,
as per Ralf-S. There is no need to stitch to solid in Alibre.
In Alibre, used the 3D Measurement tool to get square inch area of a face surface.
Just need to scale the MoI map trace to physical distance reality, before the paste, and convert the Alibre area to acres.
(Have not got arround to doing a MoI script to measure planar area of a polygon plane yet. Do not need such a script now. In my naivity, I think it is possible, just wondering about what order the sides of a closed polyline could be stored in, under different creation circumstances, and whether to extract the points by line segment, or by 2d polygon vertex?)