Since the feature is already implemented I think it should be a little bit easier to just add an option such like "Always search for selected text".
Well, thats the whole point of the loooong discussion on #758, changing the search string every time something is selected limits what editing actions can be used between searches (they have to _NOT_ select anything or you lose the search string), but if its an option on the search dialog it should be ok.
Since it already loads the selection on opening the dialog, if CTRL+F (or whatever is configured as the keybinding for search) while the dialog is is already open, loads a new search from selection without closing and re-opening, thats also an acceptable enhancement IMNSHO.
Pull requests are welcome (and don't forget to document in the manual).