You might be able to achieve this with the NotInCriteria, as follows:
new NotInCriteria
(
new CustomMetaValueCriteria
(
new CustomMetaKeyCriteria("ExtendedType"), "%", Criteria.Like
)
)
I haven't tested this, it's just a thought. Even if it works, be sure to check if it performs as well!
PS: next time, please use the tridion.stackexchange.com forum for Tridion-related questions!