The ePub format recognizes that extra metadata may be included in a document. So while the standard doesn't currently store that information, there may be reasonable ways to do so.
There's some discussion
here that talks about how to encode it in XHTML, for use in Sigil, and an attempt to future-proof the data by sticking to the Dublin Core standard.
It
may be relevant to what you're trying to do.
m a r