IIRC this is the first thread about this feature and I want to get it solved now ;-).
Basically, adding the code is not that hard. The only remaining question is how to make it accessable for the user. Nick suggested "Maybe just a keybinding to focus the search box that sets a reverse search flag, which is cleared when the search box loses focus would be enough." So, I think the keybinding would be the major way of using it. Anyway, I also think we need some kind of visible indication that the incremental search is in backwards mode.
IIRC Jeff suggested to change the background colour of the search box.
I could think of an additional button between the search box and the search button in the toolbar (even if this would bloat it a little more). This additional button would be a toggle button, which is in normal state (unpressed) if the usual forward incremental search is active, and it will be toggled(pressed) when the keybinding is pressed or the user pressed the button directly. So, you can quickly see in which mode the incremental search would work, the button indicates it. But it's just an idea.