How do I use inverse projection in gdalwarp?

Question

I have a small orthographic image I want in lat-long (equirectangular). Using gdalwarp to reproject causes a truncation at the poles above 80 degrees (feature or bug?). Does unprojecting using the -invproj option have a syntax peculiar to its nature? This command gives an error:

ERROR 1: Translating source or target SRS failed:
+invproj=ortho +datum=WGS84 +ellps=WGS84 +units=m


gdalwarp -t_srs "+invproj=ortho +datum=WGS84 +ellps=WGS84 +units=m"  EEEE.tif PPPP.tif