Experiment in kitchen chemistry well underway... Our Japanese grocery store stocks two types of Kikkoman Ponzu sauce. Kikkoman Saiguchi Ponzu (with the green label and lid) has a base of kelp & bonito flake dashi, so it's not something I can use. The yellow labelled and lidded Kikkoman Yuzuka Ponzu uses kombu dashi, but it unfortunately not available online. And since Steve has taken a liking to it... I'm brewing. So far, I can report that a mix of 2 limes and 1 lemon replicates the yuzu taste quite well. But by itself it overwhelms the dashi base.
The Kikkoman version lists apple vinegar on the label, so I'm supposing that mellows the citrus kick a little. We'll see...