Thanks to both of you.
Eric, your solution sounds nice, but I dont exactly get the part

"..before the line launching the MessChess.exe insert a copy from either your kishon_mute.lua or ..."

The original mess.cmd looks as follows:

@echo off
cd..
REM normal engine window:
MessChess.exe %1 %2 %3 %4 %5 %6 %7 %8 %9
REM hidden engine window:
REM start /B /WAIT /MIN MessChess.exe %1 %2 %3 %4 %5 %6 %7 %8 %9
cd Arena

I have no idea where to add the *.lua files

Meanwhile I will follow Franz' recommendation which is a good idea as well.
Jens