That's what Calibre is doing. I can edit the source HTML and correct typos. But nowhere as streamlined as clicking an "edit" button or better yet, clicking where the typo is, correcting it, then saving file.
Most of the time when I'm reading a buggy ePub, I don't feel like going back to Calibre, opening it, correcting, saving, exporting to iTunes, plugging iPad, then syncing again.
Well, that is exactly why I wish for an iPad app to edit ePubs! It absolutely could be as simple as clicking and typing, then pushing "save." Now we just need to find a dev who would write the app for us, lol.