Just save the unlocked endings to a file in user's save folder, then load it in the main menu to see which endings were already unlocked.
This will also keep unlocked endings after player quits the game.
Example, writing file when new ending was unlocked (remember you need to write all ending variables at once):
File *f = File.Open("$SAVEGAMEDIR$/endings.dat", eFileWrite);
f.WriteInt(UnlockedEnding1);
f.WriteInt(UnlockedEnding2);
f.WriteInt(UnlockedEnding3);
f.Close();
Reading file when menu is about to be displayed:
File *f = File.Open("$SAVEGAMEDIR$/endings.dat", eFileRead);
UnlockedEnding1 = f.ReadInt();
UnlockedEnding2 = f.ReadInt();
UnlockedEnding3 = f.ReadInt();
f.Close();
More info on file reading/writing may be found in the manual (if needed).