How to apply different filters to different parts of a video using AviSynth?
This can be done using the filter ClipClop. Here is an example script from the docs of the filter:

Avisource("D:\avs\test.avi")
ORG=Last

V1 = FFT3DFilter(Plane=0,Sigma=1.6)     # Light Luma
V2 = FFT3DFilter(Plane=0,Sigma=2.0)     # Med   Luma
V3 = FFT3DFilter(Plane=0,Sigma=4.0)     # High  Luma
V4 = FFT3DFilter(Plane=3,Sigma=1.6)     # Light Chroma
V5 = FFT3DFilter(Plane=3,Sigma=2.0)     # Med   Chroma
V6 = FFT3DFilter(Plane=3,Sigma=4.0)     # High  Chroma
V7 = FFT3DFilter(Plane=4,Sigma=1.6)     # Light Luma+Chroma
V8 = FFT3DFilter(Plane=4,Sigma=2.0)     # Med   Luma+Chroma
V9 = FFT3DFilter(Plane=4,Sigma=4.0)     # High  Luma+Chroma
V10= FlipHorizontal()                   # Flip-H
V11= FlipVertical()                     # Flip-V
V12= Invert()                           # Invert

NickNames ="""  # Psuedonyms for clips (clip index number)
    L0   = 1    # Light Luma
    L1   = 2    # Med   Luma
    L2   = 3    # High  Luma
    C0   = 4    # Light Chroma
    C1   = 5    # Med   Chroma
    C2   = 6    # High  Chroma
    LC0  = 7    # Light Luma + Chroma
    LC1  = 8    # Med   Luma + Chroma
    LC2  = 9    # High  Luma + Chroma
    FH   = 10   # Flip-H
    FV   = 11   # Flip-V
    INV  = 12   # Invert
"""

SCMD="""          # Clip editing commands in string, can also use commands in file
    C0  0,99      # Light Chroma frames @ 0 -&gt; 99
    L0  100,-200  # Light Luma frames @ 100, 200 frames ie frames 100-&gt;299
    INV 300,399   # Invert 300-&gt;399
    L0  400,499   # Light Luma frames 400-&gt;499
    FH  500,599   # Flip-H 500-&gt;599
    LC2 600,699   # High Luma + Chroma
    C1  800       # Med Chroma, Single frame
    1   900,999   # Light Luma, We used the clip number instead of a NickName
    FV  1000,1099 # Flip-V
    LC1 2000,0    # Med   Luma + Chroma, 2000 -&gt; lastframe
"""

SHOW=True

ClipClop(ORG,V1,V2,V3,V4,V5,V6,V7,V8,V9,V10,V11,V12,scmd=SCMD,nickname=NickNames,show=SHOW)


How to do it:


You define different versions of the original clip (V1 - V12) using your desired filters.
You can assign nicknames to those versions.
You define the segments using the scmd parameter of the filter. Here you can define which frame ranges of the original clip should be replaced by which nicknamed versions.