I do not believe that you can set the seed for Random Index. However, you can do a one time generation of the matrix you need and then save it to a text file. You can then read in the matrix from the text file at the beginning of your script, and set the matrix that way. Here is a simple illustration of creating a matrix, saving it to a text file, and then how to retrieve it.
x=random index(100, 10);
save text file("$TEMP/random.txt", char(x));
x=parse(load text file("$TEMP/random.txt"));
You could also create a data table with the matrix values in a column, save the data table. Then as needed open the saved table and read the values from the column into a matrix.