There is a plugin that does function replacement for F&R that already exists and seems to work well. Unlike Calibre which is written in Python, Sigil is almost purely a C++ app with a one way interface to python for handling embedded routines. So adding user supplied python functions outside of Sigil's plugin framework would be a bit of an issue as would finding the required space to add a function interface to our current Find and Replace dialog.
Since I am not a Calibre user at all. I would need to see mock-ups of how to modify our current F&R dialog (without growing its height much if at all) to add in the interface elements required by a python function replace feature.
If you can provide those mock-ups here for everyone to see and comment on, I would be happy to look at integrating that functionality into mainline Sigil.
By the way... your idea violated the first "do NOT" rule provided by this announcement!
Oh well ...