It appears that the ComboBox main label can be centre justified, maybe it would also be useful to allow justification of the dropdown text, unless this is possible already?
Possible I think but probably a bit of a pain. Jules can confirm, but I think ComboBox uses PopupMenu for rendering the menu. There's a look and feel class in PopupMenu.
It takes a bunch of parameters but none of them are justification.
So you need to knock up a LookAndFeel class containing an implementation of drawPopupMenuItem that centres the text. Just copy the code from the LookAndFeel_Vx or other class you are currently using and tweak.
Then add that LookAndFeel to your ComboBox.
Yeah, there's no easy way to set that at the moment except to just override the whole look+feel and draw the items yourself.