In Final Frontier Plus we use the normal main menu load screen, but have specified a different DDS file to show during loading, which is the time after you start a game before it switches to the map (while it is setting things up, as per the progress box that it shows over this background).
Loading image is here:
Assets\Art\Interface\Screens\Loading\LoadingScreenBGBeyondtheSword.dds
To change just the overlaid text for the main menu, you need to put a file called FF_logo_layer.dds into Assets\Art\Shared with your new logo. The original for this is in Beyond the Sword\Assets\Art\Shared (not in the FF mod, it is in the regular BtS folder). That is just the text and border above the spinning things with the rest transparent (to show the spinning things and stars).
Or, as you've been trying, you can replace the nif file in that folder (Assets\Art\Interface\Main Menu) with the one from Next War and supply your own DDS file in the same folder to replace the one named Next_War_Main_Menu.dds (size matching the one that comes with Next War is probably a good idea). NextWarSky.dds is also used since the Next War's image for Next_War_Main_Menu.dds has some transparent sections where the sky would be, so you may need this file too even if it isn't going to show up. (You could use this, or replace it with a star field or something, if you want to put some transparent sections in the top part of your Next_War_Main_Menu.dds. It is animated to flow from right to left from the viewer's perspective wrapping back to the beginning after 133.3333 seconds.)
(Looks like I The_J slipped an answer in before I finished this...)