T9 or other autocomplete function for mirc's editobox ;p
in autocomplete nicknames with tab key, should also include the words included in the visible part of active window (not the part that becomes visible after you scroll up) ;p
Fyi tabbing autocompletes what ever is in the lstbox (nicklist), so it works on custom windows as well. I guess what you say would be ok, with the cavet of course that u can switch this function ON and OFF.