The arbitrary short element codes are the ones used by another muxer ( 
files available at 
https://www.digitizationguidelines.gov/guidelines/MXF_app_sampleFiles.html#2022 
)

The support of RGBA descriptor is added, mainly by disabling in the CDCI 
descriptor related code the elements not in the Generic picture 
descriptor, and could be in a separated dedicated patch (move of Generic 
picture descriptor code in a dedicated function?).

Tested with:
ffmpeg -f lavfi -i testsrc=duration=10:size=ntsc:rate=ntsc -field_order 
bb -c:v ffv1 -level 0 test_ffv1_ntsc.mxf
ffmpeg -f lavfi -i testsrc=duration=10:size=pal:rate=pal -field_order tt 
-c:v ffv1 -level 3 test_ffv1_pal.mxf
ffmpeg -f lavfi -i testsrc=duration=10:size=1920x1080 -pix_fmt yuv422p10 
-c:v ffv1 -level 3 test_ffv1_hd.mxf