as far as I know there is no way to change the loudness value of the encoded mpeg data directly, you can only set some flags like replaygain, which have to be considered by the music player - if not then they are disregarded at all.
normalize-mp3 calls mpg123 to decode and lame to re-encode the file, in the middle of these steps it normalizes with normalize-audio (over simple bitstream wav data).
so normalize-mp3 cannot be run "independently" without normalize-audio, lame, mpg123, etc. (is my guess).