Hi OP,
I wouldn't go so far as to say that engine was designed for 5W-20, rather that 5W-20 is one of the many suitable grades, and the recommended grade in North America.
All the Honda V6 engines that I know of in Australia spec a 5W-30 oil, another suitable grade and the recommended grade here. Castrol lube guide suggest a Euro grade 5W-30 A3/B4 oil, while Valvoline lube guide suggest an ILSAC 5W-30 Dexos1 oil.
So to answer your question, yes I think your car will be fine on a Euro A3/B4 oil and I think many of those cars would already be using A3/B4 grade oils in both Australia and Europe.
BTW the standard garage / mechanic bulk fill oil here in Australia is a Semi-Synthetic 10W-40 SN & A3/B4 oil. Name any car sold in Australia, and that's most of the common cars, and half of them would be driving around on 10W40 A3/B4, most of the car owners would not know or care. They just get an oil change and drive on. Typical OCI here is 10,000KM (6k miles) or 12 months on 10W40 A3/B4 regardless of what the manual says. It's just what many people do.