http://jmsoler.free.fr/didacticiel/blender/tutor/py_trimvertex2plane_en.htm
The procedure is very simple and works in edit mode:
1/ open the Scrips window
2/ select and load the script Trim Vertices to face in the Mesh menu
3/ in the 3d window, select one face and only one
4/ push the button Original Face
5/ then select all the vertices you want to trim
6/ push Selected vertex list button
Rewritten to do not use the blender’s faces normals but the ones calculated by the script itself . It seems to work correctly now .
(Thanks to report problems if you tested this script .)
Nice, but would be very usefull if we could chose several faces. 
JMS : docteur es-python , va bientot réecrire une version de Blender entièrement codée en Python !
merci et encore bravo !
(je suis un grand fan de ton script POV)
Do you mean “ortthogonal projection” using definition of “one” plane on “several” faces ?
Thanks, soon released for python 2.4 and blender 2.40 . Next week, certainly …
(désolé, mais comme on est sur un forum anglophone, je me sens obligé d’estropier un peu la langue de Shakespeare pour la réponse) .