It seems gstreamer can do this -- see here.
Generally, as far as I know, there are 2 steps in audio normalization as currently practiced: calculation/tagging and normalization. The first of these analyzes the audio to figure out the gain values, and the 2nd step applies the discovered values to the audio data.
For mopidy, of the first step, tagging would most likely not apply, since mopidy is not made to edit files. However, perhaps it can calculate the gain immediately before playing something; that way it won't need to store the gain values anywhere, just apply them to the audio data while playing. This would only work for non-live data, though, and may not apply for album replaygain values.
The second step is of interest, to apply gain values appearing in tags of media being played to the audio data while playing.
This post about groovebasin is perhaps of interest here as far as how to use volume normalization in mopidy.