It wouldn't matter where a Japanese market model was activated. The ECG app will not work until the official firmware enabling it is released.
It is possible to buy overseas models here in Japan. I did for my previous Series 2. I purchased it while on a trip home in the USA. The downside of that was that I lost all ability to use Apple Pay locally here in Japan, no SUICA transit support, etc.
Thanks. I live in Japan, too. I got a Series 5 when I finally got fed up of my utterly inaccurate Fitbit.***
It's been great so far but there's no way the ECG has been active. My friend's a doctor who also happens to be an iOS developer on the side. He tried and couldn't get ECG to work on his Series 4. I'm pretty sure if it worked, he'd have been able to make it work. The previous comment you replied to had me confused and you clarified it nicely.
***Using Fitbit made me think I had a hormone imbalance and a sleep problem due to its inaccuracies. I was gaining weight even though I meticulously counted calories and keeping what I thought was a 600 calorie deficit. Then a friend loaned me his Apple Watch. My world changed. I wore both and realized I had a caloric surplus because Fitbit grossly overestimated my caloric burn.
Among other things, Fitbit would show exaggerated caloric burn and several extra flights of stairs climbed when I road a train/subway to work, but failed to measure the stairs I climbed to get to my third floor office (when not taking the elevator). It showed me burning up to 8 calories a minute by shuffling my feet, but only 30 calories burned during an hour of weight lifting. The worst part was that Fitbit offers expensive fitness coaching and I wouldn't be surprised at all if it's planned. The more people trust Fitbit, the more they doubt themselves. Then to fix themselves they pay loads of money for Fitbit trainers, ultimately becoming dependent on them to help save themselves. It almost seems like narcissistic gaslighting.