I have wrestled a little bit with this issue, and here's what I have learned.

1) For the pax models (-100 and -200), to get winglets you have to edit the aircraft.cfg as follows:
model=another company (or re_another company)

To be consistent you should edit the ui_type= to read:
ui_type=727-100 another company (or 727-100 another company/RE).

2) For the freighters, the display of winglets (or not) is controlled by the freighter system gauge it seems, not aircraft.cfg entries.  I got the -100F winglets to display consistently, but couldn't seem to get the -200F winglets to display at all. :(