Although it is undocumented, in python 3.4 you can control the prefix used by doing

    multiprocessing.current_process()._config['semprefix'] = 'myprefix'

in the main process at the beginning of the program.

Unfortunately, this will make the full prefix '/myprefix', so it will still start with '/'.  Changing this for 3.4 would be easy, but I don't know if it is a good idea to change 2.7.

Note that your suggested change can cause a buffer overflow.