First, there is no ISO standard for CD+G. It is only a commercial standard.
CD+G standard description can be found here:
http://www.jbum.com/jbum/sware/cdg_revealed.txt&e=747
It describes the meaning of subcodes used to encode graphical information. CD+G standard also defines interleaving and error correction and a suitable documentation is available from SONY or Phillips (I am not sure right now). But you have to pay for it.
And MP3+G is not a standard at all. It is simply a way of keeping files -- MP3 with sound and CDG with graphical information. So all of the information from the link above will apply to MP3+G.
And one more thing -- there is an Open Source project for CD+G player on SourceForge somewhere.
Hope this helps.
Krzysztof Loose
PowerKaraoke