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I think the if you want to go for threadless cogs, Halo isn't the way to go, I've heard stories of the hubs failing due to Steel sprocket fretting the aluminium hub body.
The Level cog system was awesome, not sure if its still around, the White Industries version is nice too.
I have a 6 bolt ISO setup with a Velo Solo cog onto Mack hubs. Its been on my bike for over 5 years without having to touch it.
Not a massive fan of regular threads/lockrings myself.
Keppel-
the halo threadless cogs work very well wih their hubs, they have a footplate to spread stress along the splines. if you just put a standard cog from a cassette on (it fits, as the splines are the hg pattern) then yes, it will tear through the splines, as all that fixie power braking force will be going through a 2mm or less interface.
there's alot to be said for using bike parts as they're designed to be used.
jakjakjak
I think it's a case of working out what I want from the wheels. They're start as nice summer wheels, which would lend themselves to blingy DA, but in reality they will probably end up staying on the bike full time, so I think they'll have to be sealed. I do like the idea of a threadless cog interface like on the Halos, but I could get both the Novatecs for the price of the rear Halo, and they're lighter too. Decisions, decisions.