I have been contemplating the same type of system for an eastern Tennessee project.  I have been looking for solar dealers in Tennessee period without regard to manufacturers.  I have a solar system (potable hot water) in my home here in FL.  I can say that it worked great for 20 years.  It is a Solahart (Rheem) passive system with an 80 gal storage tank on the roof.  However, this type of system is probably not feasible for areas subject to freezing as I believe that the systems are only rated to 30 degrees or so. 

I have found that I need to fire up the electric back-up a couple of days a year here in central FL.  Water always stayed warm (100-110 degrees even on the coldest days (30’s to 40’s) and we never ran out of water with three people.  Using the system for radiant I believe is possible, but a system that uses antifreeze and a heat exchanger is probably what you need to look for if you project is in a freeze prone area.  A gas back-up will probably be necessary depending on the temp of the radiant floor water. 

Here are a couple of links to  organizations that may  be able to  provide you some help:

http://www.fsec.ucf.edu/en/industry/testing/STsystems/ratings/#DHWRatings

http://www.solar-rating.org/SUMMARY/Dirsum_20070313.pdf

Where are you located?  I would like to know what you find out.