I put the equal sign in a pair of double quotes, and when passed to the command file, which runs the FINDSTR command, the command completely ignores the double quotes, and treats the equal sign as a normal parameter.

E.G. the command line 'runfindstr.cmd if @string "=" *.txt, returns all *.txt files with text "if @string =" in any of the lines.

If the command you are using doesn't ignore the double quotes, you can always put multiple versions of the command in the command file, one of which is preceded with 'if %n equ "="' (where n is the relative position of the parameter) then carry out command with a hard coded = character.