Object being attached is temporarily made semi-transparent in rendering, and transparent to clicks (unpickable).
This is working good.
As for the rest i managed to attach Draft Circle on top face of Cylinder and the same Cylinder on top face of Box. Moving the Box moves the Cylinder and Circle along with it. I added additional Box and tested the same with selecting one face on each Box and after FreeCAD restart attached Box doesn't move with the rest (Box, Cylinder and Draft Circle). But if i select the Box and start attachment editor the Box moves to the expected position. I like the fact only available options are provided and the rest is disabled but the complexity of the tool will likely contribute to some users simply not using it. This tool likely could be turned into a toolbar of tools in the future. The amount of messages (yellow/red) in Report view is overwhelming.
P.S. And i guess the tool itself is influenced a bit by Assembly capabilities currently lacking in FreeCAD and therefore try out Assembly2 ASAP as you will likely feel just like at home over there.