As a quick solution (it requires C++ though), I’ve copied the UE4 WrapBox widget and made a few minor additions to add the alignment feature.
Download this zip and extract it to your game’s source directory. After compiling there should be a new Aligned Wrap Box entry in the UMG palette, and a Horizontal Alignment property in the widget’s details panel.
I can’t make any guarantees, but so far it has worked for me.