BRU can change the characters in any one position of the string forming the name of the file of interest. This is a great option.
Language teachers can drill students by listing a number of foreign words, each in its own file e.g. aTest_word_1, aTest_word_2,... Each word in the list would appear in its order in the list. If the first character in the string were changed to a pseudo-random character, say a-z pand 0-9, then the new list e.g. rTest_word_1, 3Test_word_2, 2Test_word_3, kTest_word_4,... would be reordered when sorted.
Changing the order of the list elements would be simple for the teacher to do. Since you already can change any character in the string to another, given, character, how hard would it be to program BRU to change the character of interest in the string forming the name of the file to a pseudo-random character?