Just write your own version and put it in the MATLABPATH before the builtin one.
I DO NOT recommend you do this, however; doing that will not only give you the modified version but may cause other builtin functions that may use inv to use it instead as well and depending upon what modifications are made, either case code to error, run but produce incorrect results, or perhaps run with correct results.
If you have a special case routine that you do need for some purpose, name it uniquely and use it instead, don't mung on built ins -- and particularly numerical routines buried in the bowels of MATLAB.
