You are correct that it is in the legume family. It also purports to do some atmospheric nitrogen fixation, although I haven't found any reports as to how much.

I will stand by my statement that the bulk of the nitrogen is stored in the leaves. Here's a quote from the linked study where they are measuring the nitrogen found in the biomass. That's what biomass is, living, biological matter.

"Sunn hemp plants need not be mowed during the middle of summer. However, mowing to remove the very top of the plant can be beneficial. For example, in an experiment at Homestead, Florida, in which 5-foot-tall plants were mowed at 1 foot, 2 feet, and 3 feet above the ground and then allowed to grow 70 more days, the total yields of dry biomass were 8.3, 11.0, and 13.8 tons per acre, respectively, compared to 11.4 tons per acre in the uncut control. The corresponding numbers of pounds of nitrogen in the biomass were 259, 255, 277, respectively, compared to 238 pounds of nitrogen in the uncut control."

If you plant it as a companion crop, I'm not sure you'll get the results you're looking for. If the bulk of the nitrogen is in the biomass, you won't see any benefit until you cut it down and it begins to decay.

http://edis.ifas.ufl.edu/m/#publication?id=TR003