I was thinking about what I would do under these circumstances (may come to pass as I have a rattle from back end I have never been able to identify). I would take it to my local blacksmith/welder and have him TIG weld it for me.
I'm guessing the OP's bar has snapped inside, or near to, the damper in the middle, or possibly at the end where it turns down the trailing arm. I'm not sure any weld would hold long as this bar is subject to a lot of torsion. The bar acts like as a strengthener to the beam axle, effectively an anti-roll bar, limiting the twist on the u-shaped section, and in particular saving the welds from fatigue.
Honda garage don't know what it is?
Personally, I would go for a secondhand axle as per Jocko's original suggestion, it's not that big a job to do.
For yours Jocko, a rattle from the rear axle is sometimes the centre damper that has failed, this is just bolt on.