The concept of ReplayGain is not codec dependent, the value may even come from an external source.
I do not know how Ogg/Vorbis, WavePack or FLAC handle ReplayGain values, but in MP3 Files the values for track and album gain are stored in tags, maybe ID3, or APE, or something else.
If you can read the tags, the needed gain change could be done when the software is changing the volume.
Imho it makes sense to apply replaygain in post-processing, not in decoding.
