Can キーウィ be used to mean New Zealander? Outside of Australia and New Zealand, am I likely to be understood by Japanese speakers if I use to mean New Zealanders? Based on what weblio and Wiktionary say, I suspect not, but I want to check as sometimes dictionaries don't mention slang terms.

I would have to say no.

refers only to the kiwifruit to at least 99% of Japanese-speakers -- perhaps even more.

Those who have lived in New Zealand or Australia **_might_** refer to a New Zealander as among themselves, but that kind of private usage still would not count as an established meaning of the word within the Japanese language.