a friend of mine is building up a ss conversion and got a good deal on a campy wheel set. the only thing is, i've never seen any single speed cogs designed to go on a campy freewheel spline system. has anyone seen or heard of such a cog?

The route I went was the following:
I took Gipiemme hub, they have both Shimano and Campagnolo casette bodies. The bodies happen to fit one of Campy wheels I have (Shamal 12HPW titanium, originally intended for 8sp casettes). When I want to ride Shamal fixed, I just put my shimano casette body, home made adapter and Shimano cog of chosen size.

I also believe that there are Shimano compatible Campy casette bodies, your friend will have to check if they exist for the hub he has.

Edit: You can also use Miche single cogs intended for Campy casettes.

what about the options of a custom splined cog? does anyone know of a company who makes cogs?

if you don't mind a splined cog, just find a used campy cassette and dissassemble it, roadies throw them away every three thousand miles. I think I'm getting a new cassette at the end of week for my roadbike, if this solution will work, pm me and I'll send you the cassette.