MP3Diags and MP3Gain are not closely coupled, the user selects the normalization tool to use and there are others available - although MP3Gain is preset by MP3Diags.. So I don't think that MP3Gain should be added to MP3Diags' dependencies, any other thoughts on this?
But it would definitely be good to have such an extension, have thought of it as well. Will see if i can make it till the end of the week.
[edit: have just sent 'mp3gain' for submission]