Hi,
The thin wall electrical conduit is amazingly inexpensive stuff -- it does seem worthwhile to try and figure out a way to make good use of it.
If you used the conventional parallel riser type of collector, one feature that would allow you to use a lower temperature connection technique to connect the risers to the manifolds would be to separate the manifold area of the collector from the riser/absorber area with thin insulation, and then omit or paint out the glazing over the manifold area of the collector -- this would make the manifold are run cooler, and allow more latitude in the kind of connection you used.
There are supposed to be corrosion inhibiting additives that can be added to open hydronic systems that allow the use of iron pumps without rust/corrosion problems -- maybe the same additives would protect metal conduit. This is not an antifreeze product, but a corrosion inhibiting product.
You might try asking the people who make the Taco pumps about this -- they are pretty good about answering emails on technical issues.
I don't think that the lower conductivity of the iron would be a problem. Its far better than PEX, which does fairly well. A significant portion of the thermal resistance from the collector fins to the fluid flowing in the pipe is the slow moving layer of fluid (boundary layer) just inside the pipe wall -- once the pipe wall resistance gets below a certain point the boundary layer thermal resistance will be the largest part of the resistance, and increases in pipe wall conductivity won't help much.
If you are looking for a cost effective collector, you might also consider the MTD type collector:
Modified Trickle Down Solar Heating Collector
Gary