I haven't been to HK for almost 3 years now and need to pass through immigration to keep my permanent HKID card status.

It seems to enter HK you have to stay, and quarantine, for 2 weeks.

Is it possible to instead go through immigration while transisting to another destination, e.g. fly on to Thailand?

Thanks

Short answer: Highly unlikely

Caveat: I'm just a casual news consumer

Longer answer:
1) In normal times, theoretically you can go through immigration as long as you have enough time between flights. However, I believe arrivals get sent to get tested at Asia World Expo and then have to wait for their results, which takes at least a few hours but could be overnight depending on when you arrived and how busy they are.
2) I have no idea if you can take your negative test and just go back to the airport and exit instead of going to quarantine. Doubtful - the Centre for Health Protection's website has had a steady stream of news releases regarding the jailing/fining of people who skipped quarantine and were caught at immigration on the way to Shenzhen (e.g. https://www.info.gov.hk/gia/gene ... 2020081300646.htm).
3) I was under the impression that HK has halted transit-only connections. I think they're discussing re-allowing them for transit to certain locations, notably China, but I don't know if that has taken effect yet.