Found it! It took me a while because I assumed it would be a .bif file. 'Fog.Lbm' in the textures/Men folder has all the unit flag bits & pieces. Unfortunately I can't find a way to put the old naginata symbol on there without messing it up. Anyone else found a way?
For anyone who wants to use the old naginata symbol in the army selection screens, panelmen4, lunit4, munit3 and unit4 are the .bif files you want.