I worked on the LISP->ActiveX scripting bridge.  I discovered that CT:DEFCSTRUCT will accept the :UNION keyword (as documented on ACL page 1-3-9), but the keyword is apparently ignored.  All members are allocated at different offsets.

An Allegro CG bug:

; The following form demonstrates a problem with the TRACK-BAR control. It
; creates a track-bar and attempts to set the direction to :DOWN. When the
; dialog is opened, the direction is reset to :UP. This is because the
; CG:DEVICE-OPEN method for a TRACE-BAR-PANE receives an option list
; which has the stream options in front of the widget options. The
; stream option :DIRECTION :IO is shadowing the widget option :DIRECTION :DOWN.
(let ((item
   (cg:make-dialog-item 
      :widget 'cg-util:track-bar 
      :box (cg:make-box 78 58 124 233) 
      :direction :down 
      :orientation :vertical 
      :range (list 0 100)
      :value 0
      )))
   (print (slot-value item 'pc::direction))
   (cg:open-dialog (list item) 'cg:dialog cg:*screen* :pop-up-p nil)
   (print (slot-value item 'pc::direction))
   (values)
   )