I originally disliked this idea, particularly since it locks half the doctrine (and already rather expensive abilities for USF) behind a 60 muni upgrade, but it is far superior to the "x weapon upgrade" concept found otherwise.
Though I should say a retexture of the ranger model would probably work best for what you are going for, as it is closest to the rifleman model.
Ranger models would need the helmet of the lieutenant to make it more recognizable, plus the rangers have ranger decals on their helmet that would look out of place in a regular squad.
You could reduce the cost for sprint and flares since they would be locked behind the Veteran Sergeant upgrade, but the idea of locking the abilities behind the Sergeant was deliberate. I wanted to tie those abilities to the Sergeant for several reasons:
1. Encourage the use of the Sergeant, get the guy, get the abilities.
2. Prevent double BAR squads from sprinting up to MGs and being too strong. By making the Sergeant cost you a weapon slot you can now only have one BAR while sprinting up to a weapon team. This makes it less punishing to be on the receiving end of sprinting Riflemen.
3. Encourage use of grenade tech rather than BAR tech. Smoke, sprint and frag would be even more viable with an extra man to take damage and make aggressive grenade use less risky to try, but still expensive to pull off. This harkens back to the vCoH days of Riflemen grenade flanks being the top players US bread and butter tactic. Go watch some old DevM games from the Sunday night fights games to see what I mean.