It really makes sense for podcasts, audiobooks etc. - most of the time you have to provide an MP3 format (because all old cheap players support it) and with the ID3 chapter extension it's possible to add chapters too, which is especially useful for long podcasts.
We (auphonic.com) already implemented the chapter frame for an older version of eyeD3 (0.6.18).
However, it seems that you did quite some refactoring, so maybe this wouldn't be so easy to merge ...
Would you be interested in adding this to eyeD3?
Thanks,
LG
Georg