[Zope] ZClass property sheets and permissions

I'm working on a simple TODO list ZClass product which has a pair of property sheets, Basic and Complete, which cover the standard properties and the properties relating to when the item is completed. I'd like to bind the management of each sheet to a particular permission - 'Change TODO item properties' and 'Mark TODO Item As Complete'. So I've created those two permissions at the product level (as it appears you can't create them in the ZClass itself). When it comes to binding a particular property sheet's 'Manage Properties' permission to one of these permissions, they don't appear on the list of possibilities, yet the Add permission for the ZClass does appear. Does anyone know how I can solve this problem? John