Maybe insulate the underside of roof sheathing, water/antifreeze tubing on top of the sheathing and dark sheet-metal over that. Furring strips could create the space for the tubing.

A system like this has been on the market for several years.  I usually see it under tile roofs in Florida.

Would it work in Minnesota?

Yes, it would work.

I will have just a 4/12 pitch. Or would I be better off using pre-made panels that I set at 60 degrees, or whatever.

In MSP you would want about a 60 degree pitch, basically your latitude +15 degrees. With a 4/12 pitch I would think snow would stand on it and then it wouldn't work anyway.

But let say it was 60 degrees, how would that compare to commercially produced panels? Let just say with the same money spent.

I have no idea; the big thing with commercially available solar radiant tubes is they are vacuumed sealed tubes inside the collector. So it doesn't matter what the outside temps are, only that you have sunlight available. Putting the tubes under a roof will definitely be affected by attic temps, honestly except on a really sunny day I think you would radiate more heat out then warming up water.

Your collector efficiency is based on maximizing absorbed sunlight that is converted to heat energy, minimizing heat loss from the absorber and removal of captured energy from the absorber plate which is then transferred to the point of use. I would guess that your collector efficiency, from mid autumn through mid spring, would be less that 20% and quite often 0%. Consider the efficiency of manufactured collectors and the load that you are trying to heat before you invest in something that yields little or no return on investment.
In a nutshell, solar collectors in our northern climates REQUIRE glazing to work with the outside temperatures that make up the better part of the year. Start with what you are trying to supply heat to (domestic water, space heating, etc.) and then look for a way to provide or supplement that requirement.

http://www.carb-swa.com/documents/SolarHotWaterResearch.pdf

Have a look at the above. I have also seen this done with warmfloor used under the metal roof and flexible solar electric on top of the metal.

http://www.dawnsolar.com/

You can certainly put tubing under a metal roof in northern climates, the performance is fairly dismal, compared to most other solar thermal concepts. Dteltech is absolutely correct.
I would suggest checking out the SRCC numbers on such systems.
An unglazed pool collector would do much better, as would a glazed flat plate.
The big thing the under roof system has going for it is it is totally hidden. Unfortunately it pays a very, very large thermal price for that aesthetic.

Tom
www.americansolartechnics.com