Is it possible to create a "Vertical Perspective" projection map using ll2cr/fornav?

No. The "vertical perspective" projection, is not supported by the mapx library, and therefore not supported in MS2GT. The orthographic projection for a spherical earth is supported by mapx and MS2GT.

The mapx library is described in Points, Pixels, Grids, and Cells:
http://geospatialmethods.org/documents/ppgc/ppgc.html.