DIY solar water heating and one design uses these Stelrad-type radiators. You need ones with 4 inlet ports (one at each corner) so that you can arrange diagonal flow and then you need to paint the radiator matt black so it will become an efficient collector because white just reflects the sunlight away.
The experiments were pretty successful and on a hot day the water in the radiator rose to over 60 degrees Celsius without any insulation. That's the problem unfortunately. This collector is about 2sqm which is about a quarter of what would be needed and boxed up it would weigh too much to be mounted safely on a roof.
Anyway, this summer I'm going back to this idea, but I'm going to build a double collector using the other method from the CAE booklet, which is a copper pipe and vane design and will be less than half the weight.