After ending up with too many MP3 files without proper ReplayGain information, and knowing that mp3gain doesn't suit my needs quite right, I went jonesing around in GStreamer once again. Today was my lucky day, though; René Stadler had created a GStreamer element which calculates ReplayGain.
In what's starting to feel usual, Debian doesn't have up to date versions of the software I want to use in either testing or unstable, but this hasn't prevented the maintainer from packaging what I want to use and mirroring it in experimental. After downloading and installing it, I was good to go. First I put together a command-line example of the process, to make sure I knew what was required. Then I munged it into a plugin for Quod Libet.
There's still something fishy about it, as MP3s which I have previously found peak values for above 1.0 are now being reported capped to 1.0; Ogg Vorbis files, on the other hand, correctly exceed this cap and run presumably all the way to 2.0. I hope to clarify my understanding of this probable GStreamer bug shortly, as other than that all values match what I've seen before. I'm quite pleased the results of tonight's work.