I think Velvione may be what you are looking for. I find it's quite dry though, so you may need to add something to it to sweeten it. If you want to keep to the powdery musk vibe then Exaltolide works well in combination.
Amberlyn is the same as Ambrox or Ambroxide [but not quite the same as Ambroxan or -(-)Ambroxide] - essentially a synthetic ambergris. It has a dry, diffusive, exalting quality but I wouldn't describe it as powdery on its own. In combination with a powdery musk like velvione it might take you towards what you are looking for though.
Among natural materials I find that olibanum (frankincense) essential oil can generate that quality, though you get quite a strong top-note with that and I doubt it's the source of the powdery note in many commercial fragrances.
Hope that helps.