The new defaultIcon method in the .Application class doesn't seem to work for PropertySheet dialogs, as demonstrated in the code below.
PropertySheet dialogs are unlike other dialogs in ooDialog in that they are managed by the operating system, not ooDialog. This works, note I changed the icon file to something I have locally.
::class a subclass userdialog
appIcon = .Image~getImage('mail.ico', .Image~toID(IMAGE_ICON))
p1=.p1~new( , , , "Tab1")
p2=.p2~new( , , , "Tab2")
s~appIcon = appIcon
::class sheet subclass propertySheetDialog
::class p1 subclass userPspDialog
::class p2 subclass userPspDialog
So, the generic Application::defaultIcon does not work with a PropertySheet dialog because it is not managed by ooDialog.
Having said that, it could be made to work I believe. I just didn't think about property sheet dialogs during the implementation. Why don't you open a bug or an enhancement request for it.