Well here are the ReplayGain tagging standards:
http://wiki.hydrogenaudio.org/index.php ... n#Metadata
AFAIK, foobar2000 strictly heeds these standards and this is what I use to embed the ReplayGain tagging. Here are a few example files for testing purposes. foobar2000 recognizes the ReplayGain values and utilizes them accordingly. (Well, it should, it wrote the data!)
These have a tag attached to the MKV container with ReplayGain values:
http://cybertootie.net/downloads/RainDirectTagRG.mkv
http://cybertootie.net/downloads/GroundDirectTagRG.mkv
These have ReplayGain data embedded into their respective FLAC audio streams:
http://cybertootie.net/downloads/RainAudioTagRG.mkv
http://cybertootie.net/downloads/GroundAudioTagRG.mkv
It is important to note though that it doesn't appear that there is a standard for tagging Matroska (MKA, MKV) files. But like I said in the OP, I have tried with an MP4 video and that did not work.