vtag "MJPG" and not mjpg

Because of my NLE correct-recognize only the .avi that have as video tag "MJPG" and not "mjpg" I use:

ffmpeg .. .. .. -vtag MJPG output.avi

but ffmpeg can not write .avi with Matrox MJPEG codec (diviVcap.dll)

so I have try avs2avi:

avs2avi c:\gatto.avs output.avi -c MJPG

[#this is the gatto.avs

ffvideosource("i:\imx\c0013.mxf")
converttorgb()
crop(0,32,0,0)

]


[with vidc.MJPG=digivcap.dll in the windows registry]

but strange the encoding with avs2avi does not have costant bitrate, the bitrate lower from the beginning to the end of the clip. If I change the codec, e.g. vidc.MJPG=pvmjpg40.dll (Accusoft Mjpeg codec) all is ok, but I nedd the matrox codec because in SD I have to edit with a matrox digisuite system.

So I have try with virtualdub, with Matrox Codec: video files are ok but the tag is "mjpg" and not "MJPG".

Simply I can open the .avi file and manually change the string mjpg in the header of the avi in "MJPG" but this is unconfortable, expecially when I have to batch encode e.g. 100-150 clip files.

Is there a encoder, possibly command-line, that can do this? vtag=MJPG, considering that I have to cut the first 32lines of the source video because the .mxf IMX (sd) format have the vitc embedded in the video file) (in fact with avisynth I have to cut it with the crop(0,32,0,0))

thanks