You might put this in as an enhancement request at the Sigil site at:
http://code.google.com/p/sigil/
It might not be quite as simple as it seems since it is possible some of these options could conflict with some other code or the way the program accomplishes certain things.
Sigil is coded in some version of C, which is not the same kind of language as Python. It is much easier to add things to Calibre which is written in Python than it is for Sigil. I am not a programmer, but the above is my understanding.