Outside of Australia and New Zealand, am I likely to be understood by Japanese speakers if I use キーウィ 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.
「キーウィ」 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 「キーウィ」 among themselves, but that kind of private usage still would not count as an established meaning of the word within the Japanese language.
