It is a military group, yes... but it's not JUST a military group, is my point. The reason why they can produce units is because of their substantial civilian population. However, most civs don't HAVE an excess of civilians that will just randomly wander around... as such, it's restricted to those civs who do.
For example...
The Doviello's Hunter Clan is basically a wandering tribe of Doviello, who choose to continue living in the same way that the Doviello have for the whole of the Age of Ice, spurning the 'soft', city-dwelling ways of their companions. Although the Clan as a whole is fairly powerful as compared to the standard wandering scout, it's much more then just a raiding party... it's a community on the move.
Similarly, the Lanun Armada isn't just a big Pirate fleet. Sure, there ARE a bunch of pirate ships there... but there are many other ships, both large and small, in the group. Everything from tiny fishing boats to a huge floating drydock in included in the Armada, and as a whole, it forms a complete, self-sufficient community.
That being said, a few other related ideas come to mind, in thinking about these units that way.
First, grant the units continual XP... in a heroic sense, but without a cap. Secondly, limit them to a variant set of promotions (I'm thinking NONE of the standard promotions at all). The promotions represent the various things that can be done in the mobile city. I'd suggest...
Commerce 1-5: The first level gives +2

per turn, the second gives +3

per turn... etc... Commerce 5 negates the upkeep cost of all units in the camp's space.
Culture 1-3: Exerts a cultural pressure effect... allowing the civ to claim the square the camp is in with 1, the surrounding squares with 2, and the fat cross with 3. It's a weak effect, though; it should be almost entirely ineffective against established cities and borders.
Research 1-5: The first level gives +2

per turn, the second gives +3

per turn... etc... Research 5 gives +1 Movement.
Training 1-5: Each level gives 10% off the cost of recruiting a new unit at the Camp. Training 3, 4, and 5 grant the ability to upgrade existing units to other units of tiers 2, 3, and 4 respectively.
Warfare 1: +10% Strength, +10% Healing
Warfare 2: +10% Strength, +1 First Strike
Warfare 3: +10% Strength, +10% Healing
Warfare 4: +10% Strength, Can Heal while Moving
Warfare 5: +10% Strength, +20% City Bombard
Thoughts?