There are basically two ways to make AviSynth frameserve with subs.
1) Check the following box in CCCP settings:
And then use
DirectShowSource("C:\path\file.mkv")
In your AviSynth script.
Just checking the box shown above might do it for Super though.
2) Or a slightly harder way, but might be more accurate:
Extract the subtitle file with MKVextract, and then use the following:
LoadPlugin("C:\path\to\ffms2.dll") #Google for FFMpegSource2 to get this
LoadPlugin("C:\Program Files\Combined Community Codec Pack\Filters\VSFilter.dll") #or wherever you have installed your CCCP
FFMpegSource2("C:\path\file.mkv", atrack=-1)
TextSub("C:\path\to\subtitlefile.ass")