[-Vit-]

QTGMC will do that kind of motion blur for you (without doing any other processing):

QTGMC( "Slower", InputType=1, fpsDivisor=2, \
       ShutterBlur=2, ShutterAngleSrc=0, ShutterAngleOut=90, SBlurLimit=6, \
       TR0=0, TR1=0, TR2=0, Rep0=0, Rep2=0, SMode=0, SLMode=0, Sbb=0, NoiseProcess=0, SourceMatch=0, Lossless=0 )

1st line selects motion analysis quality, sets progressive input and reduces to 1/2 rate (SelectEven equivalent)
2nd line is the motion blur settings. Game source likely has no blur so ShutterAngleSrc = 0, ShutterAngleOut sets output blur level (0-360), SBlurLimit does what you want, will not blur regions moving slower than this value pixels per frame
3rd line switches off everything else in QTGMC

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Or if you want to do it yourself use MMask, basic idea below (assuming you will add a SelectEven() before or after):

clip=last
... # Calculate super clips, vectors fVec1, bVec1...
blurClip = MFlowBlur( ... )	   

BlurLimit = 6 # Blocks moving less than BlurLimit pixels per frame will not be blurred
motionMask = MMask( clip, bVec1, kind=0, ml=BlurLimit )
output = mt_merge( clip, blurClip, motionMask, U=3,V=3 )