I was under the mistaken impression that you were thinking of using a Surly track hub rather than the 135mm spaced MTB hub, which does have a different chainline. Either a track or mtb ss hub would work with a symmetrical dish. You would just have to work out the chainline difference up front with the crank/bb. Track hubs will give you a standard 42mm chainline which you can easily achieve up front with a SS crank such as the IRO. You can buy track hubs that are spaced to 130mm, which would also work with the IRO frame, which has 132.5 mm spacing. What crank were you thinking of using?
Heres another idea...get a ss freehub with an adjustable chainline like this (
http://www.nashbar.com/profile.cfm?c...%2FRear%20Hubs) and work out the chainline to be the same as the diskbrake mount on the other side. Then bolt a sprocket on the diskbrake mount and have a ss/fg flip flop hub.
As for solid versus hollow axles...the Novatech/Dimension/BensBikes (etc.) track hub is reported to be 270 grams. The Formula/IRO/Harris (etc.) hub is reported to be 330 grams. The only significant difference that I can see between these hubs is that one has a solid axle and the other has a hollow axle. So I would imagine you would be saving roughly 60 grams per wheel by using a hollow axle. I'm not sure if a steel skewer would weigh more or less than a pair of nuts, but I imagine the nuts are slightly lighter.
In reference to your original question about IRO stock wheels versus custom built wheels, the bigger weight savings will probably come from lighter rims (open pros), and fewer/butted spokes.