E. Kaplan wrote:
Still no clue as to how to force Beamer to make slides in PORTRAIT mode, and whether it is possible to do it for individual slides in an otherwise landscape presentation...
I don't think it's possible. Slide dimensions are hard-coded into Beamer. In the Beamer user guide, section 24.2 (which covers the beamerseminar package), it explicitly says that portrait slides (which seminar apparently allows) are not supported.