MY ideal version of this system would be something along the following lines:
BR80+ Gives you access to this system (just as they have proposed)
What this entails is, an unlocking of an NS soldier slot. You only get one of these slots per server with a BR80+ character
The soldier is a free agent, meaning it is not faction bound. However, you can only play for a faction that needs a population bump at that moment (Keeping in line with their proposed vision for the role this soldier would play.
The NS soldier has it's own progression, and can achieve all of its own ranks, unlocks ect. ect.
These unlocks would feature NS weapons (of course) but also feature iconic weapons for each faction.
Since your character floats factions, these unlocks would have to have faction parallels. The tooltips for these unlocks would feature the other faction unlocks they grant.
This does NOT enable an NS soldier to have EVERYTHING from every faction, but enough to allow you to participate in the "flavour" of a faction you may not normally main, or haven't progressed far into.
I say that the NS soldier is limited in this regard, because we don't want to undermine faction pride. Also, since the unlocks earned on this soldier also come with 2 other weapons, it wouldn't seem reasonable to give them access to everything.
Maybe we could somehow "tether" these NS soldiers to a faction specific character than granted you the prestige, allowing some (cosmetic) unlocks to bleed over into that main you've poured a few thousand hours into. This keeps people feeling like their main isn't shelved for some free agent mercenary, who just has cooler looking/functioning prestige mechanics.
My biggest fear with the proposed system, is that it shelved BR 100 characters, and undermines motivation to play them, and to represent your faction. If there is some kind of link/tether system, you could carry some of that prestige over to your Big Bad, who is essentially your personality in this game. I feel it is VERY important to retain a hold on that character who has been the lifeblood of your play experience, but this also gives you something else to do.
Even better, is when you get to double dip. Say your faction is the low pop. We'll use NC for this example. Say I have a BR 100 NC, and I love my faction, but we are underpopped, and I have access to BlackOps prestige. I can then hop on my NS soldier (who is linked/tethered to my NC main), and I have an entirely new progression to work through, still represent my faction, AND grant unlocks to my main. All the while, I have a new Soldier, with his/her own unique unlocks, who ALSO has the ability to try other factions out, when circumstances permit.
So let's say on a given day VS is the underdog in the population race. I get to fight alongside spandex, while not entirely neglecting my blue tin can. I get to earn some helmets, maybe some NS armor, maybe some camos, ect.
I'm just spitballing, and it is far from being perfect or fleshed out, but I think offers a similar experience what the devs are proposing, without undermining the feeling of accomplishment/progression on that BR 100 who has barely earned anything new in months, but you still play, because DAMMIT it took a long time to get there.