At high level the NCR emergency radio becomes much less useful, I think it would be pretty cool to see it increase in power in the late game to keep it useful. After passing a certain late game NCR story quest, the ranger who gives you the radio shows up again and gives you the new radio. He could use the same dialog and everything.

The advanced radio could have 3 options:

• 3 NCR troopers (instead of 1) but still the same toughness as the original.
  
• A ranger veteran with a trail carbine (instead of cowboy repeater, possibly with black armor but that might make it too easy to get)
  
• An elite trooper that's either in combat armor with an assault carbine or an NCR heavy trooper with a hunting shotgun.

Why not just make it so the number/awesomeness of the troopers who show up to help out depends upon your level+reputation with the NCR? I see no reason why you'd need a completely separate radio to call in more than one person.

Yes I also had that in my OP originally but figured it might actually be harder to do than make a new radio.

So yeah that's also an option if it's easier.