As Tony has pointed out, this property is READ-ONLY, including for AutoCAD 2004:
; IAcadPreferencesOpenSave:
; This object contains the options from the Open
; and Save tab on the Options dialog.
; Property values:
; Application (RO) = #<VLA-OBJECT IAcadApplication 00b5e51c> ; AutoAudit = 0 ; AutoSaveInterval = 0 ; CreateBackup = 0 ; DemandLoadARXApp = 3 ; FullCRCValidation = 0 ; IncrementalSavePercent = 1 ; LogFileOn = 0 ; MRUNumber (RO) = 9 <<<=== (RO) = READ-ONLY ; ProxyImage = 1 ; SaveAsType = 24 ; SavePreviewThumbnail = -1 ; ShowProxyDialogBox = -1 ; TempFileExtension = "ac$" ; XrefDemandLoad = 2 ; No methods