Not sure of the nearest relative as the Legume family (like all plants families is sub divided into tribes). You would have to find a member of the Caeselpinieae tribe.

I don't think however that choosing a hardy rootstock is going to make this Madagascan native any hardier though. Good luck!

I think it's unlikely, too. The frost damage is usually at the top of the tree, where the rootstock would have little or no effect.