Done. [Attached zip: mobiunpack.py for testers; patch for developers].
You probably couldn't see the problem in the html I posted even if you tried, because I foolishly neglected to use CODE tags. The real problem was an extra space character between "<a" and "filepos=".
mobiunpack doesn't say anything about "filepos=XXXXXXXX", so that must have come from KindleGen. (Although it could still be useful to warn about non-numeric filepos values).