CAD tip # 11604:

Save the code to the AllLOT.LSP file (or download this LSP file from Download), load it into AutoCAD with APPLOAD and type the command AllLOT. You can change the code from "enable" to "disable" transparency in one go (1/0).

; Set Plot Transparency ON/off (CAD Studio, 2017 - www.cadstudio.cz  www.cadforum.cz)

(vl-load-com)

(defun c:AllLOT (/ aDoc Layouts itm)

(defun LayoutTransparency (layout OnOff / xType xData)
  (setq xData (vlax-make-safearray vlax-vbVariant '(0 . 1)))
  (setq xType (vlax-make-safearray vlax-vbInteger '(0 . 1)))
  (vlax-safearray-fill xData (list (vlax-make-variant "PLOTTRANSPARENCY")(vlax-make-variant OnOff)))
  (vlax-safearray-fill xType (list 1001 1071))
  (vla-setXdata layout xType xData)
  (entmod (entget (vlax-vla-object->ename layout) '("*")))
)

  (setq	aDoc (vla-get-activedocument (vlax-get-acad-object))
		Layouts (vla-get-layouts aDoc)
  )
  (foreach
	   itm
		  (layoutlist)
; or:
;		  (append '("Model") (layoutlist)) ; incl. Model
    (LayoutTransparency (vla-item Layouts itm) 1) ;       1=ON 0=off
  )
 (princ)
)