text of "xs:override" proposal

Hello,
I have a need for overriding type definitions in one schema file by 
other type definitions in another file. Not merely extending or 
restricting, but actually replacing. For instance, I want to override a 
type defined as a simpleType as a complexType.
I found several posts on this list mentioning Michael Kay's xs:override 
proposal but can't find any pointer to the actual text.
I am curious to understand whether this feature would address my 
requirement, and whether Saxon implements this non-standard feature.
Thank you in advance,
Guillaume