> xsi:nil="true" seems like the ideal thing to use in this situation. What do you think?
Personally I wouldn't use it if I was designing the schema for this. Given that you have to specify every other element and attribute used here, it seems far more natural to just define your own markup for "unknown value" rather than bring in an external foreign attribute and the overhead of having to declare the namespace and the not inconsiderable overhead of working out exactly what the schema spec says you can and can not do with this attribute.
As to what to use instead, it depends. If the make element just takes text then using an empty element seems reasonable. If it takes values from an enumerated list of known types, "Ford", "Jeep", ... then you just need to add an "Unknown" member to that enumeration (which could be "-" rather than "Unknown" unless your enumeration is limited to XML Names.)
David
