Echo what is syntactically very different from the interrogative pronoun what. It can replace words of more or less any category, and can take on the inflectional properties of that category — cf. You bought three whats? or even They had whatted the car?
CGEL does not use any symbols indicating questionable grammaticality or grammaticality in some dialects only for those examples; therefore, the authors consider the examples correct in all dialects of English, including International English. As the echo what would naturally occur in spoken English (as opposed to written English), it is difficult to ascertain whether the examples would sound idiomatic in major English dialects (e.g. British English, American English) using Google Search. So, my question is:
Which of these are more natural in various spoken English dialects?
They had what the car?
They had whatted the car?
You bought three what?
You bought three whats?
EDIT: Or more generally, what constitutes idiomatic use of echo what in British and American English?